9. The maximum number of sales of vacant land that may be used for purposes of determining the median proportion shall be determined by performing the following operations consecutively:(1) the number of parcels of vacant land included on the roll is divided by the number of units of assessment included on the roll, and the quotient thereby obtained must comprise 2 decimals;
(2) the quotient resulting from the division prescribed in subparagraph 1 is multiplied by the number of sales entered on the basic list.
The third paragraph of section 5 applies, with the necessary modifications, for the purposes of determining the number of units of assessment and parcels of vacant land included on the roll.