Any combination of these will work in most baking recipes. Didn't know you had to be a mathematician to make brownies? It's not that difficult! Take the amount of cups called for flour in the recipe, then multiply it by 0.7. Now use that amount for your grain. Then take the number of cups of flour called for again, multiply it by 0.3 and use that amount for your starch.
The most practical way to bake fast and calculator free though, is to pre-mix a huge batch of the combination you like best. You can measure 7 cups of grain, 3 cups of starch, and throw it all in a blender. Then store it in a Tupperware container and freeze (this way it does not go rancid). Wam bam thank you Sam!