Normally I use a set of dial calipers to measure the depth of the bore on the master from the mounting face in the rear of the master, to the bottom of the bore/piston

then measure how much rod protrudes from the master's mounting face on the booster to the tip of the booster's pushrod, then it's just simple math to determine the desired length needed, plus they sell simple go/no go gauges at most auto parts stores like seen below