Yellowshark, I think you're misunderstanding how this works.
When z=0, the nozzle should be exactly touching the bed. That's what it means.
A 'z-offset' in e.g., Slic3r, is a way to compensate if you have a physical z-min endstop that is hard to adjust, and leaves the nozzle not touching the bed. I'm pretty sure that Slic3r doesn't add 0.2mm to the z-offset. That makes no sense (although Slic3r does plenty of things that make no sense, imho). Rather, the 0.2mm that you are seeing is the probably just the first layer height that you set Slic3r to use.
When you start printing, the nozzle will move to the correct first layer height, and then extrude enough plastic to fill that gap. In theory that should be plenty, if the bed is clean, and the nozzle really is touching the bed at z=0.
What George was referring to was that when you use the bed leveling wizard in the UM2 controls, it asks you to set the bed 'one paper-thickness' away from the nozzle. The firmware assumes that you have then set the bed at the z=0.1mm height, and calibrates itself accordingly so that its z=0 position has the bed 0.1mm closer to the nozzle - i.e., just touching it.
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Dim3nsioneer 558
I use 0.06mm distance with clean and cold nozzles. I measure it with a feeler or with the eye. This distance gives a very nice first 0.2mm layer with optimal adhesion. At least on my UM1.
Approx. 0.1mm ist the distance you should have with a cold nozzle as the thermal expansion of an UM1 hotend between room temperature and 200°C is 0.1mm.
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