you might be seeking a level of "perfection" in setup and operation that truly is unobtainable.
what's the problem? the caliper pistons move out and don't retract all the way?
does that leave a lot of pad drag, or is it just a matter of not returning all the way to what you thought was "perfect"?
the position of the pistons when you force them back is different from the position they will occupy after you've operated the system a few times. what you are trying to achieve is not full retraction to the point of where you can FORCE the piston, but rather, retraction to the point that you aren't getting significant brake rub. a tiny bit of drag is irrelevant to function.
it sounds like you don't really know how they work. if that's the case, if you don't understand how they operate, you can't really expect to get them set up properly on your own.