Also have a look at RichRap's
slic3r is nicer article, under the "accuracy" bit, which quite clearly explains how to calulate the ideal layer heights wrt your Z thread pitch and stepper. The difference between RichRap's and Josef's way of calulating the ideal layer height, is that Josef's new calulator ignores microstepping, which is actually a good thing. Microstepping is fine for movements, but steppers don't hold position very well on non-native steps. And the problem gets even worse when you power off your z motors in between moves.