If we're taking about 400 watts of doubel ended (HQI) metal halide, I don't think you should worry about depth too much. Assuming it is 14,000 Kelvin, I would think you could do something 36" wide, up to 24" deep, and 20" from front to back. If the light is only 10,000 K, typically you would supplement with soem actinics in power compact or T5 flourescent. Are you planning on hanging the pendant from the ceiling? That is a crap load of light, and the zoanthids aren't going to like all of it. Watch out for the heat asociated with that much light too close to the surface. I have a 150W 10,000K and almost 200W of PC actinics on my tank (36X20X20) and have planty of light with the pendant hung 12" above the tank. My chiller cycles frequently with just 250 watts total.
