Our Cut to Proof calculator (also used in Batch Actions > Cut Batch) uses TTB Table 4 to determine the end results provided by the calculator. This TTB Gauging Manual (linked below) takes into account hygroscopic exchange when adding water to alcohol.
Hyrgroscopic Exchange is a chemical property wherein ethanol molecules absorb H2O molecules. This is why when using our Cut to Proof calculators, spirit volume and H2O volume will result in a slightly lower volume than if adding H2O to H2O. Contact Support at firstname.lastname@example.org for additional questions.