I had that same question. I too am in the process of building a canopy over my 65 gallon tank (36x18x24). So I have been talking to alot of people and to my LFS and everyone basically said the same thing... 15 to 18 inches for the canopy.
Nothing lower than 15" and nothing higher than 18" (in my opinion), would start to look out of proportion!
So I'm going to have my canopy be 18" high and the lights will probably hang 1 inch off the inside of the canopy.
So the lights will be 17" above the water. (And when mounting the lights they don't have to be hanging off the canopy, but if it isn't mounted directly onto the canopy's top than that allows air to move freely aroung the MH fixture allowing the fixture to stay at a cooler temperature. It allows the air blown from the fans to cool it much faster!
Also with the fans, I know many people will say to have one on each end (one blowing in and one blowing out), but this is how I'm going to set up my canopy. I'm going to be placing 2 fans one each side blowing in and I'm going to be placing 2 fans on top blowing out, because hot air rises and with the 2 fans at the top this allows to hot air to leave the canopy faster. I'm also going to be leaving a potion of the back of the canopy open for more circulation as well.
I hope this helps!