Determined top dead center position on harmonic balancer. The outer ring HAS NOT slipped.
If i could, here's a little tip if using a piston stop device; Try to take out ALL the spark plugs or as many as possible. This will make it easy to turn the engine by hand. And try to only turn it by hand---i actually bent my piston stop device because i thought i was against compression, but it was actually the piston against the screw of the device. Fortunately, it wasn't bent too bad and i was able to unscrew it from the hole.
If you're doing it in car, then you'll probably have to use the crank pulley bolt. In this case, you want to be SLOW and GENTLE. i think if you line the factory mark up to just before the pointer tab and THEN put the device in, you'll know you're close and can anticipate better when the piston will touch the stop device.
i learned a lot from watching this video:
https://www.youtube.com/watch?v=bhIP5YKCgRE