Current year of second period of five (5) years 1st year 2nd year 3rd year 4th year 5th year and thereafter Taxes shall be current-year taxes to which shall be added the following per- centage of the difference between the cur- rent-year taxes and the base-year taxes (if greater than current-year taxes) 80% of difference 60% of difference 40% of difference 20% of difference Current year’s taxes only