Back of hand test: hover your hand over your plant, within 10-15 secs the back of your hand gets too hot then its too close. If your lights dont put off alot of heat like lower watt cfl's for example, then you can put it at just inches from the plant, but need to raise it often. If you dont have ratchet hangers on your lights then give yourself a bit of distance so you dont have to adjust every other day. All things to consider