Mine is around 40". This was chosen because it was official "bar height" which made it easy to find appropriate height bar stools. 40" also works well for a larger tank that is more commonly viewed standing up.
I figure that it is much like viewing a television. Determine what height your eyes will be at most of the time and run the horizontal centre of the tank at that height. This avoids having to look up or down to see into the tank....also minimizes distortion that you get when looking through thick glass (or plexi) at an angle.
If you are a taller or shorter than most, align to your height. It is your tank, afterall.
Easier said than done when you have only 7' of ceiling to work with, I realize.
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400 gal reef. Established April, 2007. 3 Sequence Dart, RM12-4 skimmer, 2 x OM4Ways, Yellow Tang, Maroon Clown (pair), Blonde Naso Tang, Vlamingi Tang, Foxface Rabbit, Unicorn Tang, 2 Pakistani Butterflies and a few coral gobies
My Tank: http://www.canreef.com/vbulletin/showthread.php?t=28436
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