Spandex, you mention seat adjustment several times. The idea is that people of varying body dimensions adjust the seat as required to achieve a consistent reference eye position. Seat position and body shape should not actually change the perspective if pilots are adjusting their seats correctly (although I'm sure many don't).
de facto, the question was clearly about nailing the parking line, and it was something that bugged me too for a while when changed seats. The nut on the left edge of the line seemed to work pretty well for me, but after a while you just develop the feel for it and use the force. Following a marshaller is often less precise than my own judgement.
Regarding runway centreline, which was not the original question, in both seats I tend to put the line directly beneath my ass, as it gives me the best visual reference of lateral displacement and yaw and keeps the nosewheel a smidge to one side of the lights.