In the event any judge of the supreme court, court of appeals, or superior court of the state serves more than eighteen years in the aggregate as computed under RCW
2.12.010, he or she shall receive in addition to any other pension benefits to which he or she may be entitled under this chapter, an additional pension benefit based upon one-eighteenth of his or her salary for each year of full service after eighteen years, provided his or her total pension shall not exceed seventy-five percent of the monthly salary he or she was receiving as a judge at the time of his or her retirement.