Folks,
At the risk of being flamed by Rod the Con(and recognizing, as at least some of you do, that any measurement has an error) finding top dead center by any measures that rely on the height of the piston at TDC, results can vary quite markedly.
Perhaps some of you should acquaint yourselves with common practice on high performance auto racing engines from the era before all timing being determined by precision electronics.
In short, you determine a crank position for a chosen distance of the piston from approx. TDC, say 1", you measure this distance with the piston rising from both sides of TDC, ie: Normal and reverse rotation. Then you halve that on your protractor fixed to the crank, that is the most accurate TDC I know how to measure, and far less prone to error than trying to pick the piston at the top of the stroke. If you have a nice big protractor (mine is an aluminium disk, suitably engraved, about 15" diameter) that should get you TDC +/- 1 degree.
Tootle pip!!