In counties adopting Section 31676.15, the normal rates of contribution, except for members covered by Article 6.8 (commencing with Section 31639) of this chapter, shall be such as to provide an average annuity at age 55 equal to one one-hundredth of the final compensation of members not covered by Article 6.8, according to the tables adopted by the board of supervisors, for each year of service rendered after entering the system.