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.