27 C.F.R. § 30.45
When wooden packages are to be individually gauged for withdrawal, actual tare of the packages shall be determined. The actual tare of a package shall be determined by weighing it after its contents (including rinse water, if any) have been temporarily removed to a separate container or vessel. Where the contents of packages have been temporarily removed for determination of tare, the proof, if any rinse water is added to the spirits, shall be determined after a thorough mixing of the rinse water and the spirits and before return of the spirits to the rinsed packages, and the gross weight shall be determined after the spirits and any added rinse water have been returned to the packages. In the case of metal packages the tare established at the time of filling may be used unless it appears to be incorrect. From the proofs and the net weights of the packages, the wine gallons (if desired) and the proof gallons of spirits shall be determined by the use of Table 2. However, if the spirits contain solids in excess of 600 milligrams per 100 milliliters, the wine gallon and proof gallon contents shall be determined as prescribed for such spirits in § 30.41 . If either the weight or the proof is beyond the limitations of table 2, either table 3 or table 4 may be used.
27 C.F.R. § 30.45
Sec. 201, Pub. L. 85-859, 72 Stat. 1358, as amended ( 26 U.S.C. 5204 )