This is a tricky one. When it comes to deciding how much substrate you should use you need to decide on a couple of things. If your looking to do large rocks for cave structures or large flat rocks. You will want to use more gravel to protect the bottom glass. In a tank with normal plastic plants and decorations I recommend 1 pound per gallon. So Ex. a 55 gallon tank should have 55lbs of gravel or sand.
In a tank with with large rocks you should use 1.5lbs per gallon so for a 10 gallon tank you should have 15 lbs of substrate.
Also adding a little more gravel will help you keep plants grounded even when you have fish who like to dig. Also with digging fish keep in mind what fish dig. I have a Dojo Loach that would dig in the sand I had like crazy. The Super Naturals is my favorite sand to use.