What is the maximum protein amount for a dog with kidney failure?

K. we know a dog has to be on low protein diet for kidney failure. Its not so much the protein but the phosphates that need to be low from the protein. Most dogs wont eat k/d. But not one vet or person can answer one simple question. What is the maximum amount of protein a dog can have? If k/d has 3.5 grams and kibbles and bits has 8grams of protein how can 4.5 more grams be harmful. Especially when they are on epaktin and azadoyl for protein and phosphate binders. Called a couple of university vet hospitals and to my surprise they have no answer to this. If we knew the limited amount they can take would be helpful. Any ideas anyone. Thanks