There is some science here, by the way, but it is not conclusive. The research has shown that intensely working huskies (working 3-5 hours a day) can tolerate 40-50% protein. Most vets will tell you this is much too high for house dogs. If your dogs are not active, even 30% is pushing it, as the cost is to the kidneys (which do fail in a dog's lifetime) and the protein not used to repair worked tissue is just used as calories. Something in the 20's is usually recommended, but as you see here many people prefer higher protein.
Higher protein % does not mean the protein is of higher or even equal quality, you should keep in mind. Old leather boots are very high in protein but that protein is almost unusable by a dog. Some of the foods described here are high quality -- and also high in protein.
Btw, the simple formula is this: protein and carbs are 4 kcal/g, but fat is 9 kcal/g. As adult dogs need approximately 1 gram of protein per pound of body weight, and 1/2 gram of fat per pound of body weight, a 75 lb gsd who is not working very much needs only 75g of protein and 38g of fat. (28g= 1oz.) So we are only talking about 3 ounces of pure usable protein per day! If your food is 25% highest quality protein, that means 12 oz per day -- only 1 1/2 cups! 2 cups per day for an adult female is usually enough if the food is high quality and she is not working or in whelp. Pups of course need much, much more protein per day per pound, and pregnant mothers also need much, much more.
Note that if you are using a superhigh 40% protein, then 75g comes to 8 oz or only one cup per day! Your dog is either going to starve, or she is going to get too much protein for her long term kidney health. 40% is probably dangerously high.
As dogs digest carbs poorly, you have no choice but to get somewhat high protein and fat. You don't want to get rid of fat with lots of carbs, even though that lowers calories.