The general rule of thumb says: Put your bare hand on the top of the canopy for two or three minutes while the light is on. After this amount of time, if the heat on the back of your hand still feels comfortable, your OK. Around 12 to 18 inches is normal. Cool tubes, or air cooled hoods and light fixtures with glass in them, changes everything, generally for the better.

It helps to have a cool air flow between the top of the plants and the light, with ideal temps between 70-85 degrees.