as close as possible without burning. if it isnt too warm for your hand, then it should be fine for the plants. make sure to keep moist, if you have them in small pots and close to a hps that soil is gonna dry up quick. i keep mine about a foot away but i also have a gigantic cool tube(32x42) with a 720 cfm exhaust. i can actually touch the glass and my plants dont burn but for seedlings i like a little extra distance since the soil drys up so fast.