nick stoner

I use a cool tube and my 400w is about 4-6" off the plants.

For any light you can use your arm/hand I use my forearm and place it at the top of the plant. Then lower the light. As long as you can hold your arm under the light for about 2 min. without getting a burn your good. I would start about 14" off the top and see how it goes good luck.