Makes sense to me, mark the point at which the piston engages the screwdriver say on the compression stroke, then as you cycle to piston through the power stroke you mark when the screw driver disengages the piston. So you now know that exactly half way between the two,points will be the top of the piston's stroke. Actually pretty good idea!
"We do not quit playing because we grow old, we grow old because we quit playing."
Oliver Wendell Holmes