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!
"For once you have tasted flight you will walk the earth with your eyes turned skywards, for there you have been and there you will long to return."
Leonardo da Vinci[/FONT]