The mg of phe in each gram of food is always the same. A very simple way of figuring out how much phe is in any food item is to take the weight of the food in grams and multiply it by the number in the Mg Phe/Gm food column in the phe book.
For example if you have a banana that weighs 140 g it will obviously be a bit higher in phe than a standard 114 g banana. Since each gram of banana contains 0.38 mg of phe you multiply 140 by 0.38 to give you 53 mg of phe for that banana. In the same sense if you have a smaller banana that weighs 100 g you would multiply that by 0.38 and that banana would have 38 mg of phe.