I thought Jeff was asking about clouds for an ALREADY sloped ceiling - othewise I'd have mentioned that approach, as it's a really good way to break up a full 8 foot ceiling. In fact, that's what I'm looking at doing in the bedroom that's going to rescue my sanity til I get to the REAL "toybox"...
Jeff, first off you should ALWAYS either use fire rated cloth over your absorbers or buy spray treatment to make it fire retardent.
Second, a foot of air between light and absorber is enough, unless you're using focussed infrared bulbs
Third, if you've ever played pool (billiards, sort of) you can sit at the mix position and look up - ANYWHERE on the ceiling that you could hit with a billiard shot and the next bounce would hit a speaker (including as much "english" as you could possibly put on the ball) that place should be absorbed. Same is true for the 2 foot area AROUND that imaginary impact point. Other than that, if the sound misses the absorber on the way up, it will probably pass thru it on the way back. Use some basic geometry here, draw it out if necessary -
One scenario that works is a cloud, about 4 feet x 8 feet, centered halfway between mix position and speakers, the 8 foot dimension running side to side - this could be surrounded by track lights (air gap of course) if you need that much light.
As John pointed out, I forgot to mention that a LEVEL ceiling can be improved by using plywood backed 703, purposely tilted so that it reflects sounds to the rear of the room - if you do that, in order to NOT have parallel surfaces anywhere in the sweet spot I would overlap these panels down the center of the room, with ZERO places where floor and ceiling are parallel - you can put track lighting along the SIDES , aimed to cover the areas under these tilted clouds.
In order for this to be completely effective, the plywood backed clouds should be 1 foot closer to the ceiling at the rear edge than at the front edge. This gives just over 12 degrees of splay... Steve
:)