On a 36" long tank, it would be difficult to get away with one fixture. The sides of the tank will be darker than the center 24".
I was originally going to set up a 65 gallon and I was hoping to get away with using one 27" maxspect razor, or 2 cc120p's/ 2 D120's/ 2 IT2040's.
This is not to say it wouldn't work, it just won't be as bright and you would have to keep lower light type of corals on the sides of the tank. Depth really wouldn't make a difference because you could just lower the fixture. My tank is 21"H and I'm using 1 D120 at 8" above the water level. I can't turn it up all the way because it'll bleach out stuff on the sand bed. Just my humble opinion.
Edit: my tank the D120 is on is 20" L X 18" W X 21" H just to give you an idea of coverage. The tank I'll be setting up is 24X24X20 and I want the IT 2040 but am thinking I might want the next size up. The other reason you would want two fixtures is the center brace on the tank.