§ 300. Standard of domestic distilled spirits. Domestic distilled
spirits, at a temperature of sixty degrees Fahrenheit, which have a
specific gravity of nine thousand three hundred and thirty-five as
compared with the gravity of pure distilled water at the same
temperature estimated at ten thousand, shall be deemed first proof.

The strength of any such spirits below or above first proof shall be
calculated decimally, or by the percentage in reference to such
standard, and shall be denoted as so many per centum below or above
first proof as the actual difference in strength shall be.