Keep the lights high enough to avoid burning the tops of the plant, and to avoid rapid moisture loss. A general guideline I use, is to put your hand under the light, (the back of your hand facing the light) If after 30 seconds, the heat is too much for your hand, it'll be too much for the young plants. I'd guess start at 16"-18", but this is only a guess.
The distance will change throughout the grow. Young, tender plants will need the light further away, but mature plants are heartier, and can take a bit more heat. You'll have to adjust the lights (or the plant's height) often.

In my outdoor shed, during the winter, I keep 400w lights about 8" to 12" from tops of flowering plants, but during the summer, I can only get them about 14" to 16" from the tops.
In my veg closet, my 400w MH light was kept about 24" from tops of clones and seedlings, 16" or so for adolescents, and about 12" for those about to go into flower. (same closet, same light)

Good luck with it. :thumbsup: