(a) in each year by an amount not more than the increases in taxes, payable on such project by such owner over those paid in the year prior to the expiration of the tax exemption granted pursuant to the private housing finance law, allocated to such dwelling accommodation on a per room basis based on the room count set forth in the contract with a municipality originally granting the tax exemption under the private housing finance law, and

(b) by an amount not more than the difference between the average rental per room per month last authorized by the local legislative body pursuant to the private housing finance law, and the average rental per room per month actually collected during the last year such project was exempt under such law, multiplied by the room count for such dwelling accommodation as set forth in such contract with the municipality, as well as percentage increases thereon which percentages are the same as authorized under such local laws and ordinances and generally applicable to subsequent rental agreements in dwelling accommodations in other multiple dwellings as well as any other increases authorized by law.

(3) Notwithstanding any provision of this section to the contrary, with respect to the real property of a mutual redevelopment project located in a city having a population of one million or more, the tax exemption provided in subdivision one of this section shall not apply in any year where the total period of tax exemption granted pursuant to section one hundred twenty-five of the private housing finance law and subdivision one of this section would exceed sixty years.