When making a holdover, retroactive, contract settlement or other payment to a Tier 6 member for service rendered in a plan year prior to the year of payment, the employer shall apply the rate of contribution established for that member in the year in which the payment is made or, if the member has retired, in the member's last year of service.
N.Y. Comp. Codes R. & Regs. Tit. 21 § 5015.5