I feed the 50/50 mix and havent had a problem. I dont worry about % of body weights, I just adjust my food up and down depending on what they look like, summer or winter and how much work they are doing. I have to keep them lean and muscled because they are are working dogs and expected to leap in and out of yards, back sheep etc. One of my dogs seems to require a little more than the others.
It gets back to what suits individual dogs.