I start off at about 12'' above the canopy, the let the plant grow up too 3'',4'' from top of plant. 400w , you can just about touch the plant with the bulb, they dont get real hot. but if you start to see heat stree , move them up a little. All plants differ. I had white russian last time, and i could bring the bulb 3'' from the top. now i have cali,mist going and they are more sensitive to heet