The valuation of such aircraft apportioned to this State shall be determined by the department to be the proportion of the total valuation of such aircraft determined on the basis of the arithmetical average of the following two ratios:
(a) The ratio which the total time scheduled on the ground within this State of such aircraft during the preceding calendar or fiscal year bears to the total time scheduled on the ground within and without this State of such aircraft during the preceding calendar or fiscal year.
(b) The ratio which the total mileage scheduled within this State of such aircraft operated in this State during the preceding calendar or fiscal year bears to the total mileage scheduled within and without this State of such aircraft during the preceding calendar or fiscal year.