(b) the difference betweeni. 1/12 of 25% of the average of the Maximum Pensionable Earnings for the year in which the surviving spouse first became entitled to the surviving spouse’s pension provided for in this Act or to a retirement plan payable under this Act or under a similar plan, having become entitled to the other such pension, and for each of the two preceding years, and
ii. the amount of the retirement pension of the surviving spouse.