Notwithstanding Section 20466, the approximate contribution quoted by the board and the actual contributions for a contracting agency that is an employer for purposes of Chapter 9 (commencing with Section 20790) shall be the employer rate under Chapter 9, plus the additional amount required under that chapter on account of liability for service to date of contract and for benefits with respect to which it is not subject to Section 20506, the amount to be determined in accordance with Section 20466.