just set you filament diameter on the machine to 2.68 or 2.69mm instead of the normal 2.85 and set your retraction settings to 114% of your normal settings. Keep in mind that the MK8 needs to be placed correctly. Otherwise the filament will hang on the side of the indentation. Bam you are done 
I could calculate the E-Steps but i dont know how many degrees per step and how many microsteps the machine does.
Edit: Finally found all of the info. Calculating
First, sorry for not getting back to you guys earlier. I've been busy lately and I think I've spent a grand total of 10 minutes on the forum the past few days.
Just wanted to jump in and provide some feedback after weeks of printing with IRobertI's design.
The design works well, yet I still had issues from time to time. Either because of the filament being grinded, which happened some time during retraction.
I also found out that the filament was eroding the feeder.
At the bottom of it happens when the filament is pulled into the feeder. I printed a filament guide to avoid that issue.
The grinding I'm guessing is a matter of dialing in the spring pressure. It looks like you have some pretty nice bite marks in the filament though. Try both increasing and decreasing pressure and see how it goes.
I'm a bit surprised by the amount of wear and tear you're getting at the bottom of the feeder. I just took a look at my printer and my filament doesn't touch the side of the feeder. Did you insert a piece of bowden in there?
@Nicolinux: Yeah, it's an unfortunate side effect of the open design, there's nothing to guide it into the proper location. In the original there's a pretty tight path leading to the bowden so there's no chance it can go on the side. Using the snap on guide should help. And of course straightening and cutting the filament at an angle. I guess I could decrease the entry hole underneath the bowden to get rid of the lip. It's difficult to make it perfect though without going over or under the diameter since everyone's prints can differ slightly. Then again I guess any tiny overshoot would get ground down rather quick and no longer be an issue.
I also found out that the filament was eroding the feeder.
Here is a closeup on my feeder (also at the bottom and in the same direction as Flashp0int's)
just set you filament diameter on the machine to 2.68 or 2.69mm instead of the normal 2.85 and set your retraction settings to 114% of your normal settings. Keep in mind that the MK8 needs to be placed correctly. Otherwise the filament will hang on the side of the indentation. Bam you are done
I could calculate the E-Steps but i dont know how many degrees per step and how many microsteps the machine does.
Edit: Finally found all of the info. Calculating
Ok after some test i have found 365 Steps to be very good usable.
to apply them copy this to layer 0 on any gcode:
M92 E365
M500
Note that the M500 saves this to permanent memory so if you turn your machine off and on you will still have the new value. However I'm not sure if you will lose this when you update to latest firmware.
Right...
So that happened.
I'm back to the original feeder for now. At least it worked fine for a full 3 days 
God dammit... My feeder has been sitting at full tension for ages without snapping, I don't understand why people keep snapping theirs :(
Yeah, mine is still going strong. Looking at the layers near the failure point it looks like they weren't cooling properly, might have weakened it enough to fail at that point. I added an additional fan at that point in the print to avoid that problem.
I don't know what happened either. It was printed at 100% infill. It just broke completely as I was trying to undo the latch. I pushed the arm a bit closer to the body - as to not have to pull too hard on the latch - and it just snapped. The arm wasn't even touching the body yet so it's not that I was pushing extremely hard.
God dammit... My feeder has been sitting at full tension for ages without snapping, I don't understand why people keep snapping theirs :(
It relates to bad printing Orientation, not enough strength in between the layers or a too big carbonization of the used material .
Thinking about this a bit more I'm thinking that perhaps we could print the arm at an angle. My thinking is that then the shearing forces wouldn't be along the layer lines but slightly against them if that makes any sense. Thoughts?
Or maybe I should just give up on the aesthetics, make the back completely flat and have it printed on its back...
edit. Takei snuck in first And yes, I realise that the orientation is incorrect for strength, it's just that I've never had any trouble with it and I'm stubborn 
Seeing how it broke that did occur to me as well... I'm going to try to print the arm again with simplify3d, there I can determine at which angle the infill is printed. I'm thinking their default setting of -45º and 45º would be a good idea as it is as far from perpendicular to the forces as can be
Thinking about this a bit more I'm thinking that perhaps we could print the arm at an angle. My thinking is that then the shearing forces wouldn't be along the layer lines but slightly against them if that makes any sense. Thoughts?
Or maybe I should just give up on the aesthetics, make the back completely flat and have it printed on its back...
edit. Takei snuck in first
And yes, I realise that the orientation is incorrect for strength, it's just that I've never had any trouble with it and I'm stubborn
Printing at an angle could introduce new problems. I would go with the easiest approach and give up on aesthetics. That would be the most efficient way to go. I never gave a shit about aesthetics. It is a nice cherry on the top, but if it introduces problems, takes to much extra time or makes stuff more complicated than it has to be just go plain easy and simple
Couldn't the hex shaped hole be rotated 30 degrees, leaving thicker walls at that point?
@Takei: Actually now I remember that I also considered that the strength of the mounting hole of the arm would be too weak if the arm was printed on its back. The tweaked version now has a 3.5mm hole instead of a 5.Xmm hole but it's still not a hole lot of material holding it together, certainly less than the area that snapped here.
@Cyan: The problem I had with that orientation was that the top of the hole would cave in a bit and prevent the nut from slipping in there. To compensate I had to make the hole bigger and then we're pretty much back to where we are now as far as wall thickness goes.
If only we had a few more millimeters to work with... Maybe moving the knurled sleeve out a bit and replacing the original mounting screws with longer ones could work.
F- it, I'll just tweak it to be printable lying on it's side, that should be the best of two worlds I think. Will probably have to leave the area where the latch goes open for printability though.
I'm a bit surprised by the amount of wear and tear you're getting at the bottom of the feeder. I just took a look at my printer and my filament doesn't touch the side of the feeder. Did you insert a piece of bowden in there?
Yes, inside. It does not stick outside,.. maybe it should.
I now moved the spool below the desk and the printer. The filament gets straight into the feeder. I'll check over time if this works.
I also have to do a few tests with PET and Nylon soon for a project. Maybe I'll try print the feeder in one of those materials.
I've printed myself https://www.youmagine.com/designs/ultimaker-2-power-cable-filament-guide pretty much the first day:
I think it really helps as the plastic is going in almost straight up.
This thing is nice but it relies on the fact that the printer has direct contact with the table. I wouldn't do that. Maybe add some cushioning feet to reduce vibrations.
You should also consider printing a spool-holder with a smaller diameter shaft, which results in less friction with the spool due to better leverage, or make Robert's ball-bearing version.
@Nicolinux: I don't think I've had any problems following the fact that the printer is directly on the table. It's the first I hear about cushioning. Has this been tested/proven? I'm all about getting better prints, as everyone is probably
I've printed myself one of these pretty much the first day:
I think it really helps as the plastic is going in almost straight up.
From friction point of view, I don't know if straightening the filament actually helps - as it's not going to be straight in the Bowden tube in any case. In my opinion the original filament path in UM2 is not optimal as the knurled side of the filament is on the "outer track" in the Bowden tube. Furthermore, I got inconsistent results depending on how full/empty the reel was.
My solution was to place the filament reel on the left side of the printer:
Now the knurled side of the filament stays on the "inner track" in the Bowden tube. Also, the curve of the filament on the reel continues quite naturally after the feeder. I've been thinking about adding some extra guides for the filament, but I haven't yet seen there enough need for that.
Just for the record i have Robert's feeder on for a couple of weeks (at least 8 i think) and i had no broken parts at all and i change filament once a day at least.
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some of us need a small tutorial. I have just bought a MK8. But I am quite lost.
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