27 CFR § 21.31 - General.
(b) Proof of spirits for denaturation. Alcohol of not less than 185 proof shall be used in the manufacture of all formulas of specially denatured alcohol, unless otherwise specifically stated or unless otherwise authorized by the appropriate TTB officer. Rum for denaturation shall be of not less than 150 proof and may be denatured only in accordance with Formula No. 4.
(c) Use of Denatured Spirits. Users and manufacturers holding approved Forms 5150.19 (formerly 1479–A) covering use in processes or manufacture of products no longer authorized for a particular formula may continue that use. Pursuant to written application and subject to the provisions of 26 U.S.C. Chapter 51, part 20 of this chapter, and this part, the appropriate TTB officer, may authorize the use of any formula of specially denatured alcohol or specially denatured rum for uses not specifically authorized in this part. The code number before each item under “authorized uses” shall be used in reporting the use of specially denatured alcohol or specially denatured rum.
(d) Analytical tolerance. TTB will apply an analytical tolerance of ±5% and use standard rounding rules in determining whether specially denatured spirits complies with the formula prescribed in this subpart (or in accordance with § 21.5).