The optimum touchdown is at 1,000ft, not 1500 ft, at least for airliners. Noting your "A 'perfect' touchdown is one that occurs at the 50% mark (i.e. 1,500ft from the threshold)." is the reason I'm seeing low percentage touchdown zone points, when landing ON the 1,000ft mark (where you are supposed to land). This is something that should definitely be looked at.