To derive phe from protein: multiply protein by about 30 (for fruit), 40 (for vegetables) or 50 (for grains like flour, etc.). The label gives only a rounded protein amount, so to be more safe in your estimates, you should add 0.5 to the value on the label. For example, if you have a food label from a baked product where the first ingredient is flour and the protein is 1 gm per serving, you would multiply 1.5 gm protein by 50 (=75 mg phe) to get the maximum amount of phe this product probably contains. If you have a food label from a fruit-based product that says 1 gm per serving, you would multiply 1.5 by 30 (=45 mg phe) to get maximum amount of phe. If you are using an exchange system, you would need to convert phe to exchanges.
This is a gross estimate of the phe content of a food. To be more accurate, you would need to have protein content of the item to one decimal place rather than the rounded figure.
(this came from the Listserv - from Virginia Schuett - thought it was a good thing to have on file)