I think most people will tell you that depth is personal preference for style you're looking for. I personally only added enough sand to be visually appealing to me. Functionality wasn't important and I did not opt for a DSB. I only used about 40lbs of sand in my 5 ft 120. I set the rocks on the glass and filled it where I could reach. Admittedly, only small portions of sand made it behind the liverock but you can't see those areas of my tank.
I used special grade dry sand when I set my tank up. I advise not using a livesand version to prevent the dead matter from having a shot at causing a mini cycle.
In my tank, my return is nominal for flow. I run a sicce 3.0 and it's only about 4-5x turn over for my tank. All my flow comes from two pp-15s at opposite ends of the tank. I'm running them on constant flow at level 2. Even at this level, the center of my tank has a bare spot. Reef mode and pulsing causes sand storms.
