If I'm doing display alignment on a compact Mac, I find it easiest to cut a sheet of white paper to the correct image size. Just hold the paper up to the display and adjust the image to match the size of the paper sheet. It also helps confirm that the geometry is correct (parallel top/bottom and left/right sides).

Metal rulers can mar the display surface. Plastic rulers solve this, but, are still too long to fit flat against the CRT.