The only way to really know what pressure your pump is putting out is to put a gauge on it. Adding, or removing shims, is just a guessing game. And yes, I've done that more than once.
You can build a pressure tester pretty cheap with an old hose, a gauge, a couple of fittings and a ball valve. I tried using the parts store loaner pressure tester once, but that POS leaked so bad it was totally useless.
Production tolerances when the pump is manufactured is one reason why some valves have more or less shims than another. That Borgeson Saginaw style pump, like virtually every pump and steering gear, are made in China now.