Wolfpup, measure what you are feeding by weight when you add the first time and see how much of your scoop it fills.
Then whichever is the lightest, call that "one unit" and figure out how many units of each you add. (As in if you have 1/2lb of oats be your lightest and you add 3lbs of Ayers feed that would be 7 units. One unit of oat, six of layer.)
Then you can calculate your feed percentage by weight. So like if your units are 1lb, 3lbs of 16% layer + 1lb 12% wheat, 1lb 14% oats and 1lb 22% fish food....
Is 96%/6 units is 16% protein. Its actually simple averages. But measure by weight not volume.