Each such subscriber’s share of the deficiency for which an assessment is made pursuant to this article, shall be determined by applying to the premium earned on the member’s policy or policies during the period to be covered by the assessment, the ratio of the total deficiency to the total premiums earned during such period, upon all policies subject to such assessment. For the purpose of this section, premium earned is to be calculated upon the basis of the amount of the consideration for which the policy is issued, without the deduction of attorney-in-fact fees, policy fees, or other charges, excepting charges which do not recur upon the renewal or extension of the policy.