I've got 3 600W air-cooled HPS lights. I've been told 18-24 inches. Right now I have them at 24inches above. What do you think? Go 18? or just try 24 out, see how the plants react and how temps are from the distance then adjust if necessary? I previously had them about 30-32 inches above the plants and at the plants height the temperature was getting up to 78F.