If you are weighing foods, use the mg of phe per gram of food this is the most accurate, as someone else said, this never changes (unless a products ingredients change).
Weigh the banana. Let's say it is 115 grams.
Take the banana weight and multiply by the mg of phe per gram of food to get the phe in the banana.
115 g of banana X .35 mg/g = ~40 mg of phe in the banana.
We have found large discrepencies in many foods between the listed weight for a serving versus what something you have may actually weigh. No two portions of food are exactly the same. We no longer estimate and always weigh our son's food.