Disconnect battery for safety. Insert the device in no1 making sure the piston is down. The length of the plunger is unimportant, but you want the piston to stop "down a ways." If you buy one rather than building one, make sure you get a lock nut on there to stop the stem from wobbling around
CAREFULLY wrench the engine around until the piston stops on the device. make a temp mark under TDC on your pointer tab, onto the balancer
Rotate the engine opposite direction do same thing. This will result in two temporary marks, and halfway between is TDC
If the original mark is correct, that is where it will be. This is one I built for SB Mopars in the '70s. Had it all these years