Not very difficult if you don't try to oversimplify things, Just work with a set of ranges and its very simple, i.e. 1-2, 2-4, 4-6, 6-8, 8-10 and so on if i want to get that defined, If not i go 2-4, 4-8, 8-12, 12-16 and so on in 4" increments between zones or areas and it works quite well, Even when you throw in the caveats of , "or more" in the favored elevated and upslope areas in some cases.