FWIW:
I have a 55G, and only have at the max 4" in the left half of the tank.. the right half is at most 3".. and some of the mid sections (front to back mid) where the rock is, only about 2 inches..
I do a 10 gallong water change every 2 weeks, and test with Sea Chem test kits.. and my nitrates seldomly rise about 5 - 7..
So.. I'm actually quite happy with an overall less than 4". I wonder if the sandbed is actually doing anything, or if its the water changes keeping the levels low.
BTW, the sand is HD Southdown, and I have an Aqua-C remora skimmer, with the overflow box and bubble eliminator.