I measure the depth of the piston recess in the "thimble" & subtract the amount the round nub protrudes from the booster flange from that dimention but someone posted about having a helper work the pedal & you looking in the M/C & seeing how much travel you have before the compensating port in the floor is closed by the piston cup. Might need clear fluid or to drain most of the fluid then adding clear fluid (have not done it so just thinking out loud). At rest the piston needs to move a hair before it starts building psi but I'd think that'd be more of a caliper drag issue rather than the booster being triggered but to k.i.s.s. I'd check it & I'm assuming the booster worked good on the donor car it was on.