The trouble with this hobby is if you make a mistake, you're usually f*ucked up back to square one!

So play it safe. Let the seeds sprout in darkness. once you see the hook emerge from the soil, then its time to introduce them to light.

Seedlings will die in as quick as 3 hours, if too close to the light to some degree. (First hand experience)

Start at 24" and inch your way down everyday. Remember, this is far more "ART" than it is "SCIENCE" so play it safe. Inch down each day until you have a feel for your plants, their appearance and growth rate.

Too close too fast and you will stunt their growth. If you were using cfl, you could make the jump faster, but with the 400HPS, play it safe and watch the plants.

I dont buy the place your hand where the plant is and 'feel' for heat. Seedlings are far more sensitive to light than my hands. Watch for stretching...if you get any, drop a couple of inches but no more...

You'll find the height.