How high should a 400w MH be?

[CHEAPREEF]

Hi guys, just wanted some opinions on how high off the water surface a 400watt Iwasaki should be. I'm finishing off my new hood on tuesday and need a rough idea where my mounting brackets should be. The hood will contain 1 400 watt plus 2 overdriven NO actinics to 80 watts each. It will be a free standing hood with open areas all round. It's going over my 75gal with a 15" water depth (afetr 4" DSB). I was thinking 8" From water surface to bottom of MH, what do you guys think?

Thanks
Clinton