In any county or district, subject to

Section 31676.17, 31676.18, or 31676.19, that adopts or has already adopted the provisions of this article, the retirement allowance of members subject to

Section 31676.17, 31676.18, or 31676.19 shall be computed according to either the provisions of subdivision (

a) or subdivision (

b) of this

section as selected by the board of supervisors. (

a) The retirement allowance for service rendered prior to the effective date of this

article as specified in the resolution mentioned in

Section shall be computed in accordance with the provisions of

Section 31676.17, 31676.18, or 31676.19, as applicable. The retirement allowance of any member with respect to service performed after the effective date of this

article as specified in the resolution mentioned in

Section shall equal the total of the following:

(1) The fraction of one seventy-fifth of the first three hundred fifty dollars ($350) monthly of the member’s final compensation set forth in the table appearing in

Section 31676.17, 31676.18, or 31676.19, as applicable, in the column applicable to his or her age at retirement taken to the preceding completed quarter year multiplied by the number of years of creditable service as provided therein.

(2) The fraction of one-fiftieth of any remaining portion of the member’s final compensation set forth in the table appearing in

Section 31676.17, 31676.18, or 31676.19, as applicable, in the column applicable to his or her age at retirement taken to the preceding completed quarter year multiplied by the number of years of creditable service. (

b) The retirement allowance shall be computed according to the provisions of

Section 31676.17, 31676.18, or 31676.19, as applicable, and federal old age and survivors’ insurance coverage shall be on an additive or supplemental basis.