(a) Proof. Except as provided in paragraph (b) of this section, when the proprietor is required to gauge distilled spirits, the proprietor must determine the proof in accordance with the procedures prescribed in part 30 of this chapter, Gauging Manual.

(b) Use of Initial proof. After a proprietor has determined the proof of distilled spirits in accordance with the procedures in part 30 of this chapter, a proprietor may use the initial determination of proof when required to make a later gauge at the same plant. However, a proprietor must determine the proof again when:

(1) A bottling tank gauge is required by §19.353;

(2) A gauge for tax determination is required by §19.226; or

(3) In any case where the proof may have changed.

(26 U.S.C. 5559)