I made a piston stop that bolts directly to the deck. At first I just used a dial indicator on the piston itself, but quickly realized that there was about a 3 degree (guessing) period where the piston didn't move. So for best accuracy, I made the stop. I'm not sure exactly how far down in the hole the piston was stopping, but I know that it was hitting the stop 34 degrees on either side of TDC. Hopefully that made sense...