Curious observation panic . . . but why would one need to infer piston movement from plughole-rod movement using trigonometry?
Your initial point is spot-on - the further down the bore one can set the piston-stop the more accurate the bisecting calculation.
BTW . . . this technique will not work on a stovebolt:
the spark plug angle is so acute that a threaded rod will contact the opposite side of the combustion chamber.
