To calculate the rate reduction to be made four years after a rate case as required by Section 367.0816, F.S., the following methodology shall be used. The annual amount of rate case expense, which is equal to one-fourth of the total allowed rate case expense, shall be divided by the regulatory assessment fee gross up factor. The resulting number shall then be divided by the revenue requirement to determine the percentage of the rate reduction. The percentage is then multiplied against the new rates to determine the amount of the future rate reduction. Revised tariff sheets implementing the reduction shall be filed no later than one month before the end of the fourth year.