Why not just use a tdc finder and degree wheel? Using A10 stroke=84mm=3 5/16 inches=53/16 x 2 for a complete revolution, 3.4 degrees= 1/16". Don't recall what the advance should be in inches, but a conversion to degrees rotation should be easy enough, and simpler than trying to ensure that you are truly perpendicular to the piston? Am I missing something?