When I put a standard square up against the TV, I got a number within a sixteenth of an inch. That's better than "coarse," and good enough for me!

It's odd that, while you slam a square as being coarse, you suggest that just looking at a photo might tell me where the holes are. Meanwhile, I'm looking for the exact mounting height, not just what you can approximate without using ANY measuring tool.

If I had a photo but no electricity I'd use the method you suggest. I'd use a divider instead of a compass, but hey, details.

If I had electricity (and I often do), I'd use Paint to display the photo, make it reasonably large onscreen, count pixels and do some arithmetic (it doesn't even rise to the level of being called math) to determine the needed dimensions.

By the way, did I mention that the strangest thing happened? This 65" went up where we had a 42" plasma, putting the TV exactly where we wanted it, without moving the bracket at all!