The tax rate for counties levying a sales tax under section 67.505 shall be computed by:

(1) Dividing the amount of the sales tax revenue required for reduction under subsection 3 of section 67.505 and section 163.087 by the total assessed valuation of the county and multiplying by one hundred to determine the amount of property tax reduction; and

(2) Subtracting the property tax rate reduction in subdivision (1) of this section from the tax rate ceiling for each class of property or subclass of real property.