If on the basis of the experience as reported by an insurer under section 3852 the benchmark ratio since inception (ratio 1) exceeds the adjusted experienced ratio since inception (ratio 3), then a refund or credit calculation is required. The refund calculation shall be done on a statewide basis on a refund calculation form in a format prescribed by the commissioner for each type in a standard medicare supplement benefit plan. For purposes of the refund or credit calculation, only experience on policies issued within the reporting year shall be excluded. A refund or credit shall be made only where the benchmark loss ratio exceeds the adjusted experienced loss ratio and the amount to be refunded or credited exceeds the minimum level. The refund shall include interest from the end of the calendar year to the date of the refund or credit at a rate specified by the secretary of health and human services, but not less than the average rate of interest for 13-week treasury notes. A refund or credit against premiums due shall be made by September 30 following the experience year upon which the refund or credit is based.