Upon the discontinuance of service, as herein provided for, the company shall promptly refund to the customer the pro rata amount of every advance payment for any service after said discontinuance, said refund to be based upon the relation of the period after the discontinuance of service to the entire period for which said advance payment was made, after deducting the proper charge for any water consumed.
N.Y. Comp. Codes R. & Regs. Tit. 16 § 504.3