All insurers which are members of the Association shall participate in its writings, expenses, servicing allowance, management fees and losses in the proportion that the net direct premiums of each such member (excluding that portion of premiums attributable to the operation of the Association) written during the preceding calendar year bears to the aggregate net premiums written in this State by all members of the Association. Each insurer's participation in the Association shall be determined annually on the basis of such net direct premiums written during the preceding calendar year, as reported in the annual statements and other reports filed by the insurer with the Commissioner.