Mine is at 12" from the surface of the water, and my tank is 24" deep.
The reason you get many different answers is because not all tanks have the same height or inhabitants.
Hope this helps a little..
Also has to do with what youre keeping and where you want to keep them. I've found that softies, lps & ricordias like it closer to the bottom so as not to have too much light strength on them I keep my lights 6" off the surface and my tank is a 75 gallon.
There are so many factors concerning this becouse there are a lot of variables. The basics for this is the depth of your tank, but more importantly what are you housing in your tank. Sps, softies combo ? this makesa tremendous difference. In addtition, the reflector, bulb, and ballast combo are most definitely not created equall. I used to run a luminarc reflector which i would hang 10" from bulb to water and get the par measurements i wanted..now i am running a lumen bright reflector, coral vue ballast and reeflux bulb and i have to hang them 16" from water to bulb and getting the same if not better in many instances par readings.