Personally I would back off on the light when they are still so young. A couple feet away when starting seedlings or clones....I actually go with T5 or even CFL when they are that small until they fill out and have more root growth. The back of the hand method is blaaa, I see people give that advice all the time. With a 400 not as big of an issue, I have an air cooled hood and my I can touch the glass with the back of my hand and it is not too hot. Would I lower my lights 2-3 inches from the plant nope! Even though heat is the main issue everyone talks about and is worried about there is also something else you need to consider. Light saturation, you can burn your plants not from heat but to much light. I keep my 400 around 12" away sometimes more....you can try and play around with it but plants will react different. Bleaching of leaves can occur (and can look like a nute deficiency), you may notice leaves folding downward, and the opposite if you see the leaves point up it may be to far and stretching toward the light. There is a happy medium and your plants will thank you for it.
