Mrcrowley it's right in the middle of both those points. It's at the highest possible point the piston can travel, with the crank and rod all aligned vertically. Typically, the marks on the crank, et cetera, are considered "good enough" for guys like us. It becomes far more important when you have very tight tolerances (e.g. high compression pistons, small combustion chambers, high lift/duration cams, and for degreeing the cam). In the case that you're trying to determine the "absolute TDC" you'd use a dial gauge, and make the determination via those measurements and some math.
In short, I would just trust the marks on the engine itself for a stock build.
That said, what exactly are you trying to do? I can't really tell by your posts/questions.
