In case the board elects to avail itself of the assessments referred to in Section 12897 it shall, on or before the first week day in September, or if such week day falls upon a holiday then on the first business day thereafter, fix the rate of taxes, designating the number of cents upon each hundred dollars, using as a basis the value of property transmitted to the board by the county auditors, which rate of taxation shall be sufficient to raise the amount previously fixed by the board. These acts by the board shall constitute a valid assessment of the property and a valid levy of the taxes so fixed.