This is what I always preach, good topic. I have my 400 about 4 inches away from the little ones in veg with an air cooled and with the air cooled on the 1000W I go about 10 inches, I could go closer, but I don't want to loos the ft print and I have noticed that some plants can't handle the lumen intensity and the chlorophyll in the bud will become bleached and turn white.
Adieu