In cases where the board finds that a taxpayer is being assessed disproportionately with respect to other properties within the same city or town, the board shall compute the measure of damages in the following manner:

1. (1) by computing an equalized tax rate by dividing the total taxes as assessed for that city or town for the year for which the finding was made by the fair cash value of the city or town, which shall in no event be higher than the equalized value as finally reported to the general court pursuant to section 10C of chapter 58 for that city or town in the year next preceding the year for which the finding was made.
2. (2) by applying the rate as computed in accordance with subsection 1 to the fair cash value of the property and thereby determine the taxes which should have been paid.

3. (3) by subtracting the amount of taxes which should have been paid from those actually paid or assessed.

   In such cases within a city or town that has been certified for classification by the commissioner under the provisions of section 56 of chapter 40 and has implemented such classification system the measure of damages shall be computed on the basis of the tax rate of the class in which the property has been assessed or should have been assessed, as the case may be.