California Code, Government Code - GOV § 21052

A member or retired former employee who elects to receive service credit subject to this section shall contribute, in accordance with Section 21050, an amount equal to the increase in employer liability, using the payrate and other factors affecting liability on the date of the request for costing of the service credit.  The methodology for calculating the amount of the contribution shall be determined by the chief actuary and approved by the board.  A member or retired former employee electing to receive service credit for service subject to Section 21076, 21076.5, or 21077 shall pay the contributions as described.