How high to hang my metal halides?

I had a 6x39 t5 fixture that I'm switching to 2x250w. Using 2 icecap ballast with 2 20k DE hamilton bulbs. Where should I start to hang and how should I adjust so I can eventually get to 6-10". Seems like overkill for a 40B but I plan on having clams and lots of sps. As well as some Lps at the bottom. The way I'll hang is that I'll screw in a 2x4 facing vertically into the top frame of my stand. And then a horizontal 2x4 that faces out over my tank. Then have picture hanging wire hanging from those so my light is adjustable.