The amount which the board of county commissioners shall levy as the county tax shall be computed by adding together the amounts of the annual appropriation and of that part of any special appropriation which is to be raised by taxation and deducting therefrom so much of the probable receipts from all sources, except loans, and so much of the unappropriated balance in the county treasury at the close of the auditor’s books for the previous year as the board deems advisable. The board, on or before the October meeting required by section 11-11-05, shall determine the amount of taxes that shall be levied for county purposes and shall levy all such taxes in specific amounts.