Your minimum thickness should match your material specs. If the zi manufacturer says .5 mill at a minimum .5 so it doesnt chip or cause weakness during sintering or layer9ng if applying porcelain, especially if you have less than ideal margins on the prep.
Your gap thickness will largely depend on your mill, cam strategy, and preferences on fit. Most people start with the default and then tighten or loosen it slightly.
I run 2 different cam software's and milling machines so my specs allow for good fits with either workflow. But I have a dialed in setting for milling abutments on one mill only as the tolerance in ca ity side finishings better and more precise.