Pulling V.A at idle

It makes zero difference how far down the piston stop goes. But once you screw it in and adjust it, don’t move it again. So, rotate the engine over by hand till the piston in that cylinder is down. Put the stop in. If adjustable, all that gives you is two marks closer together, so screw the adjustment on the stop about halfway and leave it. Rotate the engine one direction by hand slowly until the piston touches the stop. Mark the balancer on zero. Now rotate the engine the other direction until the piston touches the stop again. Mark the balancer at zero. Exactly between your two man made marks will be true TDC. If the two marks are too far away from each other to accurately measure then screw the adjustment on the stop out a little and try again. Easy peasy.