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.
New York Legal Code