Congratulations on your new baby girl. My name is Breanna. I am eighteen, and like your daughter I have PKU. I can imagine you are probably going through some difficult emotions at the moment, but I reassure you in the end everything will work out.
A lot of PKU treatment involves guess and check. There is no simple equation to determine how much breast milk your daughter needs, because everyone with PKU is different. Some people are able to tolerate a lot of phenylalanine while others cannot tolerate much. Eventually as time goes on you will be able to get a better idea of how much phenylalanine she can have, and it will not be as much guessing involved.
As far as her phe levels go the measurments that your doctor is using are ummol/L. Some people us mg/dl in which case phe levels need to be between 2-6 mg/dl (to convert from mg/dl to ummol/L you simply multiply by 60). If you need anything else please feel free to let me know.