Is this with the return in the middine and the fuge on a side, or what is the setup? If that were the case I would put the fuge as high as you could (maybe 16-18") so you can get a good 5-6" DSB and have room for chaeto and/or other macros.
Most skimmers work between 8-10" of water so look at your skimmer and see how deep it likes to be. You could still go 15-16" and then support it on an eggcrate stand or something to get the proper depth. If you're going to use some controller for ATO, you will put the float in the chamber with the return pump since that is where the evaporation takes its toll.
Don't forget to set it up so that in the event of a power failure, the sump can hold all the water is has to. I drilled a small hole in my return PVC as a siphon break at the 90degree elbow inside my overflow. It shoots water out, but mainly water comes out of the return nozzles. If the power fails, the water does not get sucked down into the sump, instead I get a few bubbles in the display when power returns as air is pushed out of the line.
