Motor vehicles acquired by the owner after the commencement of the registration year shall be proportionally registered by applying the “New Mexico mileage percentage”, which is the figure resulting from the division of in-state miles by total fleet miles used in the original application, for all of the fleet vehicles for the registration period to the regular registration fees due with respect to the added motor vehicles for the remainder of the registration year. The registration fee for additional motor vehicles shall be prorated on a quarterly basis.