Determination of the percentage of annual increase or decrease in the cost of living shall be made by the retirement system manager on or before April 1st of each year by reference to the current U.S. Department of Labor, Bureau of Labor Statistics Consumer Price Index for the San Francisco-Oakland area for the whole of the preceding calendar year. The percentage by which such index shall have increased or decreased over or below such index for the full calendar year immediately prior thereto shall be the percentage used to calculate adjustments in retirement allowances pursuant to Section 2.124.1690 of this chapter. (Prior code § 34.09.901)