KatieG...that\'s exactly the method I use.
Sometimes I use a slightly different mix for a rockier look to the ground. I mix 25% Fine Ballast, 25% Medium, 25% Coarse, 12.5% Fine *Talus* and 12.5% Medium Talus. Talus is similar to ballast, but it has a more uneven texture...DO NOT use talus by itself on a base...everytime I\'ve tried it looks like crap.
Note: I have a container, and I use a 1 Tablespoon measure for the ballasts, and a half Tablespoon measure for the talus...so you can measure the 25%/12.5% thing easily that way.