The allocation of the cost of an insurance policy provided for in Code Section 45-9-40 among the various departments, boards, bureaus, agencies, and instrumentalities of the state shall be in accordance with the equitable distribution thereof as shall be determined by the Department of Administrative Services based as nearly as practicable upon the proportionate exposure to liability which each such department, board, bureau, agency, or instrumentality bears to the aggregate exposure to liability of all such departments, boards, bureaus, agencies, or instrumentalities of the state.