a) if you are within a millimeter (or so) it really does not matter (visually)
b) any lock you put on the primary will be "setup" while you are using the scope. So you use the scope normally and get it focused, Then you move the locking bolts until they touch the backside of the mirror, while looking through the focuser, so you and see when the bolt touches the mirror*. Do this 3 times, and then the mirror is locked in place, and you can <gently> tighten down the holds(=nuts) on the bolts.
c) astrophotographically, you do as in (b) but watch that test images have equal sharpness in all 4 corners.
d) after locking the mirror "down" you can go through a collimation procedure to dial in the final degree of sharpness (not needed very often if already collimated)
(*) when the bolt touches the backside of the mirror the mirror will be tweaked a few thousandths of an inch which is easily seen as image shift.