No miscellaneous revenue from any source shall be included as anticipated revenue in the budget in an amount in excess of the average of the amount actually realized in cash from the same source in the next preceding fiscal year, and that actually realized during the current fiscal year to the time of preparing the budget plus that to be received in the remaining months of the year estimated as accurately as may be, unless the Mayor shall determine that the facts clearly warrant the expectation that such excess amount will actually be realized in cash during the budget year and shall certify such determination in writing to Council.