Your brewers grain supplier/salesman should be able to answer your question.
I think you are under estimating water content, I've seen loads over 70% moisture and it varies from one brewery to another.
If they can't supply you with a test result grab about 2 cups worth and send it to a lab. My "book value" has dried brewers grain at 21% protein with 91% dry matter. So if your cubes are 20% protein and the brewers grain is 50% then roughly 500 pounds of brewers grain is about 250 pounds of cubes. Certainly not exact but ballpark and that is based on protein alone. There is a whole lot more to animal nutrition than just crude protein