AR guy:
All fired primers get flattened to some degree.
The way cartridges work is the firing pin hits the primer, setting the primer off, which eventually ignites the powder. The primer when it first ignites, pushes the primer slightly out of the primer pocket and is stopped by the bolt face. The gas pressure expands the case, pushing it back against the bolt face and reseats the primer. So the primer is propelled against the bolt face and then jammed hard against the bolt face as the cartridge case moves rearward under gas pressure to reseat the primer.
The higher the pressure the cartridge case works at, from 10,000 psi (some pistols) to 60,000 psi (some rifles), the flatter the back of the primer is reformed. So primer flatness is not a good measure of case pressures (nor is the cratering around the firing pin impression on the back of the primer cup), as there is always some of it to some degree. About the only way to make use of the primer flattening is to compare it to a commercially loaded fired case.
But if the primer is spit out of the case or the primer pocket becomes loose (you detect that when you try to put a new primer in the case at reloading time) or the head of the case separates from the body then you are way over pressure.
The safest thing for pistol rounds is to load in the middle or to just above the middle of the manual's load range. If the gun operates at that powder level then the chances are the accuracy is better than at a full load and it is easier on the gun.
LDBennett