Mileage proportions for a fleet not registered in this State as a fleet during the preceding year must be determined by the Department with which the vehicle is registered upon the application of the applicant on forms to be supplied by the Department, which must show the operations of the fleet during the preceding year in such detail as the Department may require and the estimated operation in this State in the current year or, if no operations were conducted during the preceding year, the estimated operation in this State for the current year.