C. The surtax rate on coal shall be increased on July 1, 1994, and on July 1 of each succeeding year by an amount equal to the product of the dollar amount of the severance tax imposed on each ton of coal by a percentage equal to the percentage rise in the producer price index for coal from the calendar year 1992 to the calendar year just prior to the year in which the surtax rates are computed, but in no case shall the surtax rate be decreased. The rates so computed shall be computed by the department in April of 1994 and in April of each year thereafter and published on or before May 1, 1994 and on or before May 1 of each year thereafter. If the producer price index for coal is substantially revised or if the base year used as an index of one hundred is changed, the department shall make an adjustment in the percentage used to compute the surtax rates that would produce results equivalent, as nearly as possible, to those that would have been obtained if the producer price index for coal had not been so revised or if the base year had not been changed. If this index ceases to become available, then a comparable index based upon changes in the price of coal shall be adopted by the department by regulation.