N.Y. Comp. Codes R. & Regs. tit. 20, § 8186-9.3 – Computation of base percentages, current percentages, and current base proportions.

(1) Determine the decrease in taxable assessed value in the class or portion class for parcels being removed from the class due to a change in class designation between the 1989 and 1990 assessment rolls.
(2) Determine the increase in taxable assessed value in the class or portion class for parcels being added to the class due to a change in class designation between the 1989 and 1990 assessment rolls.
(3) Divide the increase in taxable assessed value determined in paragraph (2) of this subdivision by the special assessing unit class change in level of assessment factor between the 1989 and 1990 assessment rolls.
(4) Subtract the decrease in taxable assessed value for the class determined in paragraph (1) of this subdivision from the quotient obtained in paragraph (3) of this subdivision.
(5) Divide the difference obtained in paragraph (4) of this subdivision by the 1989 taxable assessed value of the class or portion class. Add one to the quotient to determine a class market value adjustment factor.
(6) Multiply the class or portion class market value obtained pursuant to section 1803-a(2)(a)(1) of the Real Property Tax Law by the appropriate class market value adjustment factor obtained in paragraph (5) of this subdivision.
(b) Current percentages.
The current percentage for each class and for each portion class shall be determined pursuant to section 1803-a of the Real Property Tax Law.
(c) Current base proportion.
The current base proportion for each class and for each portion class shall be determined pursuant to section 1803-a of the Real Property Tax Law, provided that the sum of the current base proportions must equal one and each current base proportion must be expressed as a percentage to at least four places to the right of the decimal point.

The legislative body of each special assessing unit shall compute base percentages, current percentages and current base proportions for each class and for each portion class as provided herein.

(a) Base percentage.

The base percentage for each class and for each portion class shall be determined pursuant to section 1803-a of the Real Property Tax Law, provided that the estimated market value of each class or portion class is adjusted to account for any change in the class designation of parcels. The estimated class market value shall be adjusted in the following manner: