17. Estimates. The Engineer shall from time to time as the work progresses, on or about the last day of each month, make in writing an estimate, such as he shall believe to be just and fair, of the amount and value of the work done and the materials incorporated into the work by the Contractor under the specifications, provided however that no such estimate shall be required to be made when, in the judgment of the Engineer the total value of the work done and the materials incorporated into the work since the last preceding estimate is less than —— dollars. Such estimates shall not be required to be made by strict measurements, but they may be approximate only. The Contractor shall not be entitled to demand from the Commission as a right, a detailed statement of the measurements or quantities entering into the several items of the monthly estimates, but he will be given such opportunities and facilities to verify the estimates as may be deemed reasonable by the Commission. When in the opinion of the Engineer, the Contractor shall have completely performed the contract on his part, the Engineer shall make a final estimate, based on actual measurements, of the whole amount of the work under and according to the terms of the contract, and shall certify to the Commission in writing, the amount of the final estimate at the completion of the work. After the completion of the work the City shall pay to the Contractor the amount remaining after deducting from the total amount or value of the work, as stated in the final estimate, all such sums as have theretofore been paid to the Contractor under any of the provisions of the contract, except such sums as may have been paid for extra work, and also any sum or all sums of money which by the terms thereof the City is or may be authorized to reserve or retain; provided that nothing therein contained shall affect the right of the City, hereby reserved, to reject the whole or any portion of the aforesaid work, should the said certificate be found or known to be inconsistent with the terms of the contract or otherwise improperly given. All monthly estimates upon which partial payments have been made, being merely estimates, shall be subject to correction in the final estimate, which final estimate may be made without notice thereof to the Contractor, or of the measurements upon which it is based.