N.Y. Gen. Bus. Law § 300

§ 300. Standard of domestic distilled spirits. Domestic distilled\nspirits, at a temperature of sixty degrees Fahrenheit, which have a\nspecific gravity of nine thousand three hundred and thirty-five as\ncompared with the gravity of pure distilled water at the same\ntemperature estimated at ten thousand, shall be deemed first proof.\n The strength of any such spirits below or above first proof shall be\ncalculated decimally, or by the percentage in reference to such\nstandard, and shall be denoted as so many per centum below or above\nfirst proof as the actual difference in strength shall be.\n