All members of the association shall participate in its writings, expenses, servicing allowance, management fees and losses in the proportion that the net direct premium of each member, excluding that portion of a premium attributable to the operation of the association, written during the preceding calendar year bears to the aggregate net direct premium written in this state by all members of the association. Each member’s participation in the association shall be determined annually on the basis of the net direct premium written during the preceding calendar year.