I have 2 x XB200 from HGLED in a 2.5' x 5' area - I've kept them every where between 24" and 4". 8-14" seems to be the sweet spot. 14-16" or so when allowing them to stretch and staying around 8-10" during maturation in flower.
The spectrum on these boards is way different though. It's 6 colors that end up making a more white light than you'd expect. Under a camera lens, it ends up looking more pink/red than it appears to the naked eye.
Either way, this is my first run with LED and I'm not fully convinced quantum boards are really the way to go. I think I was getting more output with my 315w CMH bulbs. Time will tell. Flowers are certainly being produced though.