There are 2 factors that favor using a piston position as far down from TDC as your tool can reach:
1. piston motion is much greater per degree of crank rotation as you approach 75° ATDC = better accuracy.
2. the stock stovebolt piston pin is offset 5/64" (.078") and will have asymmetric motion before vs. after TDC during the first period of rotation. This error vanishes as the piston retreats down the cylinder.
