inconsistent Z height

So I had my machine calibrated and printing really well at .3 layer height. When I set it to print at .2 (first layer still .3) my first layer had gaps between the threads, like the nozzle was to far away. No other settings changed. I shouldnt have to play with the Z tab screw when i switch layer heights right?   

So anyway i played with the Z tab till i got the threads squished just right, printed a calibrration box and the dimentions are all out of wack. the bottom of the box should have been 2.0mm and its measuring 1.83mm.

why is this happening ? R host should make these calculations so that i dont have to play with ztab right?

another quick question, what is the biggest layer height/widest perimeter I can get with a .35 nozzle ?