I use HF for my seedlings and have been very happy with it. As soon as they are old enough I switch them for Fox Farm Oceans Forest (usually at about two weeks of age) but Happy Frog has worked very well for me for brand new babies.

Your 400w mh on the other hand........several feet at first. I actually have a 400 amongst some of my lighting and it burned the brand new leaves right off my seedlings from two feet away. So at least three feet or so at first; then work it down slowly.