The commissioners of the county commission, the clerk of the county commission the assessor, the collector and the treasurer of the county shall constitute a board of estimate of anticipated revenue. In each year, after the tax levy shall have been made by the county commission said board shall make an estimate of the revenues of the county for the current year; provided, however, that such estimate may be made at any time during the year prior to the making of such tax levy upon the basis of a tentative levy made by the county commission, but if the estimate shall in due course be thereafter changed or such levy shall be changed when made at the time provided by law, then such prior estimate shall be changed and corrected accordingly to conform to the facts, and the amount of the notes to be subsequently issued shall be limited or may be enlarged to conform to such subsequent or corrected estimate, so that in no event will the aggregate of all notes issued exceed ninety percent of the revenues as finally so estimated. In determining the percentage of the taxes which will be collected for the current year, the board in making said estimate will use the average percentage of collections of general county taxes of the prior three years.