(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. (1) A bottling tank gauge is required by § 19.353;
2. (2) A gauge for tax determination is required by § 19.226; or

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

   (26 U.S.C. 5559)