I won't be sloping front to back but may have some small mounds and valleys. Figured I'd have 4" in some spots while others may have only 2". I calculated for 3" if it was flat across the tank so bought an extra bag. I sure didn't want to run out while in the middle of re-doing the tank. Don't think the fishy's would like that. It's going to be stressful enough for them as it is. Plus I can always use the extra in another tank.
Keeps us informed how the KH does if you don't mind.