I know at the moment you and your wife are probably scared and confused, and that is normal. Most parents who have children that have PKU do not know what it is until they hear the diagnosis. Eighteen years ago my parents were in your position. Today my mother is a pro when it comes to PKU! Everything I know I learned from her, so I can tell you from personal experience that as time goes on PKU will slowly become more and more apart of your daily life.
The measurement your doctor is using is micromoles of phenylalanine per liter of blood or ummol/L. When your daughter has a blood test done the doctor is measuring how much phenylalanine is in her blood. People who have PKU need to keep their phe levels between 120 and 360 ummol/L. Each persons situation is different. Some people who have PKU have more enzyme activity than others, which means they can tolerate more phenylalanine without having high phe levels. The only way you will be sure if 17 oz of breast milk is too much is by giving it to your daughter and doing a blood test. If her phe level comes back high your doctor or dietitian will adjust the amount breast milk she is allowed to have in order to lower her levels. Unfortunately treating PKU involves a lot of guess and check.
Your daughter will be just fine. As time goes on you will be amazed when she begins to roll over, sit up, crawl, walk, and so on just like other children her age. When I was born my parents did not have the internet to turn to. They were not sure what my future would hold, but today I am in college studying Nursing and Pre-medicine! Five years from now I plan on attending medical school to become a medical geneticist.