25 centimeters. 1 cm per every 10 watts. Also, you can put the back of your hand on top of the plant. If its too hot for your hand , its too hot for your plant
it depends on many factors. do you have an air cooled hood? how strong is your ventilation; about how many times a minute is the entire volume of air in your box changed out? it could be as much as 12" or as little as 2".
Well.. i have a wooden closet 5 feet wide 6 feet tall and 3 feet deep. i have to smaller 6 cm diameter holes near the bottom (one left one right) for passive intake and one 12cm cm diameter hole at the top with 220volt fan with a rpm regulator for active outtake. i had a smaller one but i had a tremendous temp problem so i installed this little beast. My city's temp at this time of year reaches 33 degrees Celcious but when i boost the fan i manage to have a 32-33 degrees in the closet.
Well i think the answer of, as close as possible while not burning you is good. I mean makes sense, if you hand can handle it , ......if it can't the plant likely can't. My question to add to this is; I have a 15" plant and my 70w hps is only about 2-3" from the top of the plant........that said the light is a good 12" + from the smaller leaves and branches on the lower part of the plant. How do you solve that situation? Or just leave it be and let the branches stretch toward the light? Another lower light?
by training the plant to grow flatter or at least get more growth up by the light by either by topping, tyin it down or ScrOGin it.