The best and most accurate way to find TDC on an engine that is already assembled is to use a piston plug and a cheap easy to make timing wheel. This is how we time aircraft within half a degree. Find compression, insert a plug into the spark plug hole, turn engine till piston touches plug (gently) mark timing wheel, turn opposite direction until piston touches plug (gently) mark timing wheel, and divide the two marks mathematically down the middle, remove plug and turn to derived mark. Presto TDC compression of number one. Then look at your timing mark on the flyweel and see how far off it is. Generally the deeper the plug you can thread in the spark plug hole, the more accurate the results.