I think you may be trying to place too much control with protein levels. I use Masters Meaty Crunch (23%) and control my dog's weight by volume. The dog's metabolism determines how much food is needed. My old Bethany passed her food through very quickly and needed relatively large amounts of food just to keep a little flesh on her. With Anna on the other hand, what goes in stays in! Literally, by Bethany's standards she is on starvation rations!
All changing foods means (Once you have found one you like) is that you have to start all over again finding the right amounts for your dog. I seriously wonder sometimes just how Anna manages to live on what I feed her but she is completely tireless and can spend a day in the field working without turning a hair.
Count your self lucky, cut the amount of food, pocket the savings and put the saved money towards a nice summer holiday in the sun!

Regards, John