You can subtract a measured leftover portion proportionally only when its composition is reasonably represented by the batch average. Residue that is mostly oil, sauce or a particular ingredient may not have that average composition.
The uniform-leftover model
Imagine a fictional fully mixed batch weighing 1,000 g with 1,500 calculated calories. If 100 g of the same uniform mixture remains uneaten, that portion represents 150 calories under the model. The remaining 900 g accounts for 1,350.
When the model fails
A pan with mostly separated cooking oil is not the same mixture as the completed food. Multiplying its weight by average batch density can misstate what was removed. You need a supported component amount or a clearly stated uncertainty, not a guess justified by a convenient total.
Keep served and eaten records distinct
The batch recipe total describes what was prepared under its ingredient assumptions. The served total describes what was portioned onto plates. The eaten amount may differ again. For a household recipe, do not claim all three were directly measured if only ingredient weights were recorded.
The example is original arithmetic with invented quantities. FDA supports the portion basis and USDA supports matching material descriptions. A transparent estimate can be useful without claiming exact intake. If the residue composition is unknown, preserve that limitation instead of forcing the numbers to reconcile to a falsely precise consumed total.
Sources and example boundaries
- USDA: Foundation Foods documentation
Food descriptions, portions, missing nutrients and sample variation.
- FDA: serving information
Serving sizes and household/metric units on U.S. labels.
All food figures and prices in the worked example are invented to explain the method. They are not restaurant nutrition, tested recipes, shopping prices or personal dietary advice. Use the current label for the product you actually have.