I'm designing a bookcase for a room with tall ceilings. It'll be 88x31", with a depth of 11" in the lower half, and 7" in the upper half. Does that seem sufficiently deep to avoid tipping worries? (It'll be secured to the wall, obviously, but I want it to be safe on its own.) It'll have a substantial back board (either 1/2 or 3/4" plywood), and it seems to me that the reduced depth in the upper half should move the centre of gravity toward the back when loaded. Should I be looking at a deeper bottom section than 11"?