The board of trustees shall adopt rules to exclude from annual compensation all or part of salary and wages in the final years of a member’s employment that reasonably can be presumed to have been derived from a conversion of fringe benefits, maintenance, or other payments not includable in annual compensation to salary and wages. The board of trustees shall adopt rules that include a percentage limitation on the amount of increases in annual compensation that may be subject to credit and deposit during a member’s final years of employment.