27 C.F.R. § 19.411 – Recording gauge.

(a) Receipts into storage. When a proprietor receives into the storage account packages of spirits from customs custody, the proprietor must use the last official gauge to compute and record the average content of the packages received in the storage records required under § 19.590. That gauge also will constitute the basis for entries on the package summary records required under § 19.591. If the last official gauge indicates a substantial variation in the contents of the packages, the proprietor must group the packages into lots according to their approximate contents and assign a separate lot identification to each group of packages, based on the date the packages were received on bonded premises.
(b) Receipts into processing. When a proprietor receives into the processing account packages of spirits from customs custody the proprietor must determine the proof gallons of spirits received in each package. The determination may be made by using the last official gauge.
(26 U.S.C. 5232)