To compute the amount of motor fuel that a motor carrier uses to operate commercial motor vehicles on highways in the State, the motor carrier shall: compute its average fleet miles for each gallon of motor fuel as a fraction: the numerator of which is the total miles that the motor carrier’s entire fleet travels in all states; and the denominator of which is the total gallons of motor fuel used; determine the total miles the motor carrier’s fleet travels on highways in this State; and divide the number computed under item (2) by the fraction computed under item (1) of this section.