I agree w/ the hand method. or better yet use a thermometer. if you have a thermometer shoot for 75-70 degrees F. If you don't have one use the hand method. if you feel good heat off the light w/ you hand just above plants then it's too close. If you don't have a reflector & cooling duct fan hooked up then you should. because you will then be able to get your lights even close while keeping everything cool enuf & plants will recieve more lumens. you lose lumens over distance so the closer the better until heat becomes an issue. like i said b4 75 degrees is great. 90+ will kill. So just saying 3 feet without knowing what kinda cooling if any you have set up + the ambient air temp of the outside room & other factors Isn't good advice imho but @ the same time it is likely that 3 feet is a "safe" distance.