253.39. The amount of abatement is equal to the difference between the amount of the tax mentioned in section 253.36 that would be payable were there no abatement for the fiscal year considered in respect of the eligible unit of assessment and the amount that would be payable if the amount of the tax were a fictitious amount equal to the amount of the tax for the preceding fiscal year as determined pursuant to the first two paragraphs of section 253.38 and increased by a percentage corresponding to the eligibility threshold fixed by by-law for the fiscal year concerned.
The by-law may, however, provide that the percentage of increase of the amount of the tax in respect of a unit of assessment to establish whether the unit is eligible for the abatement for the second fiscal year to which the roll applies and to calculate the amount of the abatement is determined by using, instead of the amount of the tax for the preceding fiscal year, the fictitious amount applicable at the end of that fiscal year, account being taken of alterations made to the roll in respect of the preceding fiscal year.