The value of unused stores and supplies on board a vessel at the time of loss, and for which claims for loss will be paid equals:

(a) The value of such stores and supplies on board at the completion of the previous voyage, plus

(b) The value of stores and supplies purchased and placed on board the vessel before the commencement of the voyage during which the loss occurred, plus

(c) The value of stores and supplies purchased and placed on board the vessel after the commencement of such voyage, less

(d) That portion of the sum of paragraphs (a), (b), and (c) of this section which was sold, transferred, used or consumed to, but not including, the date of the loss.