The best way to micro-manage the flipper power (assuming they are too strong) is to adjust the EOS switch on each flipper. If you give it a big gap (that is, the EOS switch opens much sooner), it will decrease the power of the flippers. This is a really easy change to make, totally reversible, and takes but a few seconds to adjust.

For example, on Bally Wizard, we like a really strong right flipper (to make that long loop shoot to the top.) So we put a 28 volt gottlieb 5141 coil on the right flipper. This is WAY too strong for a 50 volt game! but by adjusting the EOS switch, we tame it down so it's the right power level. There's probably a 1/2" gap when the flipper is fully engaged!