The lesser retirement allowance under an optional settlement elected at retirement for law enforcement members whose retirement is effective prior to October 1, 1965, and who are entitled to receive benefits under the federal system, shall be actuarially equivalent to that part of the retirement allowance that is subject to option pursuant to Section 21451 that would have been payable at retirement had no optional settlement been elected after taking into consideration the reduction in the allowance provided for in former Section 21252.10, as amended by Chapter 1657
of the Statutes of 1971. Upon the election of an optional settlement by a member who has not attained the federal retirement age the board shall estimate the federal benefit upon the basis of information then available to it. If the death of the member should occur before he or she has attained the federal retirement age payments under the optional settlement elected shall be as estimated. If the member attains the federal retirement age, the board shall then recalculate the lesser retirement allowance payable under the optional settlement upon the basis of the age of the member and the beneficiary on the effective date of retirement, actuarial tables then in use and the federal benefit.