If you're concerned at all about the visual appeal of the stand/hood, I recommend mocking something up out of cardboard so you can visualize how the final product will look.
My last hood (running MH) was 24" tall on a 24" deep tank. Frankly, it looked odd. I built it so that the light rack could raise up on pulleys inside the hood. That saved a lot of burnt hair on my arms by getting the MH out of the way to service the tank. The hood was tall to allow the mechanics to happen. Very convenient, slightly ugly.
My opinion: I recommend having some convenient way to be able to move the lights enough to allow easy access to the tank. If you're like me and can't keep your hands out of the water. Plan the hood dimensions around that while trying to keep the hood from being too massive and still big enough to not cuss every time you need to mount a frag.
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