But isn't this resolution eliminated by the manual-leveling procedure? A M3-thread has a pitch of 0.5, so one tenth of a turn will give you 50 microns height difference.
Edit:
On magnets-positioning:
You just place a magnet behind the Hall-sensor (so, both on the print head) and the just measure the flux as a ferromagnetic heatbed is approaching.
Described here:
http://www.hall-sensor.biz/
quote:
"Mounting a Hall sensor and a magnet in one device gives new possibilities. Non-magnetic ferromagnetic objects can be detected, and distances measured. Several metals have ferromagnetic properties and can be used, including some Stainless Steel versions. To be able to use strong magnets for a large range, and to be able to see very small changes, the Hall sensors must also show a good sensitivity for small changes in a strong magnetic field."