The notes herein authorized shall not be issued until after the anticipated revenue for the year has been estimated, as herein provided in section 50.110, and when issued shall be in proportion to the total estimated revenue as follows: Not to exceed ten percent in any one month in any year and the total of such notes issued shall not exceed ninety percent of the total anticipated revenue in any county in any one year, but if said notes, or any thereof, shall not be issued within or at the times so fixed, they may be subsequently issued to the amount so limited, but in no event shall said notes be issued if there be on hand general revenues sufficient to pay the general operating expenses of the county.