If you put a baby pad under the saddle and ride in it, ultimately what you should find afterward are a clean dry area over the spine and wither, and the rest of the pad should be uniformly wet. It should kind of look like a butterfly for lack of a better shape. It's best to leave the horse a little dirty so you can see what areas are more unbalanced. For example, a saddle that's heavier in the cantle will produce a saddle pad that is dirtier in the back than the front.
You just have to see your distance...you don't have to like it.