If the board elects to avail itself of county assessments, it shall, before September 1st, fix the rate of taxes, designating the number of cents upon each one hundred dollars ($100), using as a basis the value of property as ascertained from the equalized assessment rolls of the county or counties. The rate of taxation shall be sufficient to raise the amount previously fixed by the board. These acts by the board are a valid assessment of the property and a valid levy of the taxes so fixed.