In the event that the total amount of investment earnings of the plan for any plan year exceeds the sum of the total amount required to credit all employee and employer accounts at the minimum interest rate for the plan year plus the administrative costs of the plan for the plan year, then following the determination by the board with respect to additions to the Gain and Loss Reserve described in subdivision (c) of Section 26202 the board may declare by means of plan amendment an additional earnings credit for the plan year with respect to the employee and employer accounts of the plan participants.