The scale of assessment upon the several classes of land returned by the engineer and viewers shall be in the ratio of five, four, three, two and one; that is to say, as often as five mills per acre is assessed against the land in "Class A," four mills per acre shall be assessed against the land in "Class B," three mills per acre in "Class C," two mills per acre in "Class D," and one mill per acre in "Class E." This shall form the basis of the assessment of benefits to the lands for drainage purposes.