There's no set answer to this due to a number of environmental factors. The BEST way to determine light height is to put your hand at the plant tops with the light at the desired height and then just wait a few seconds. Is it warm? Is it hot? If you were the plant in the container, would you want to sit under that hot of a light source for 12 hours or would it be uncomfortable? If you think it's hot, the light needs to go up. You want them to be well light, but warm and comfortable. Warmth is good, hot is bad. Layman's rule of thumb says probably about 12"-14" away from the tops, but I would always use the hand test method.