Thank you @Ridgerunner . Not that I didn't know the answer or anything ....If you mix equal amounts you will get 26 + 9 = 35. Divide that by 2 and you get an average of 17-1/2%.
That assumes they eat equal amounts of each. With scratch they usually don't, scratch is usually in big grains, not ground up together and put back together so it stays mixed like crumbles or pellets. So they pick out their favorites and leave the rest.
If you can feed it in a way that it is eaten equally it works. That's your challenge.