IMO stick with the higher percent protein as the birds will use it. I have never used the grower finishers. THat is for when growth rates plateau and is the only feed. THere might be some waste of protein with t he 20% but on the other hand you will maximize the growth.
I dont put on a lot...
I have raised a few muscovy. Put 26 in the freezer last month.
I process the same as chickens, and use a utility knife with a new blade.
I raised my birds free range with a mother duck. THe feed was 22% protein and they could eat bugs and worms, and grass at will.
I have 2 more groups...