There's an old trick for getting the screen's V to H proportions just right:

Change the desktop pattern to the standard Apple gray - which is actually alternating black & white pixels, diagonally arranged. Then hold something with a known 90˚ corner angle up against the screen - the corner of a floppy disk or a sheet of paper, say - with the corner pointing up, so the two sides slope down 45˚ on either side. You will be able to see quite clearly whether the alternating pixels line up diagonally along the sides of your test object.