Being a noob to the forums and not also having a lot of growing experience im sure there are more qualified people on here to answer your question but so far I have learned the best distance for your lights is to hold your hand under the light at the top of your plants for a minute. See if your hand feels any discomfort from the heat and the adjust accordingly.
Actaully the light can heat stress and the leafs do dry out and eventually turn brown. As said the common method is hold your hand under the light at top of plant for a few minutes and if your hand is not discomforted the plant won't be either. I personally think human skin is tougher then a plants and try to only feel some warmth not actual discomfort.