Third, the taxpayer shall ascertain the percentage which the gross revenue of the taxpayer derived from sales made and services rendered in the city is of the total gross revenue from sales and services wherever made or rendered during the period covered by the return.

    (1) For the purposes of this section, "sales made in the city" means all sales where the goods, merchandise or property is received in the city by the purchaser, or a person or firm designated by him. In the case of delivery of goods in the city to a common or private carrier or by other means of transportation, the place at which the delivery has been completed is considered as the place at which the goods are received by the purchaser.

     The following examples are not all inclusive but may serve as a guide for determining sales made in the city:

    (a) Sales to a customer in the city with shipments to a destination within the city from a location in the city or an out-of-city location are considered sales made in the city.

    (b) Sales to a customer in the city with shipments to a destination within the city directly from the taxpayer's in-city supplier or out-of-city supplier are considered sales made in the city.

    (c) Sales to a customer in the city with shipments directly to the customer at his regularly maintained and established out-of-city location are considered out-of-city sales.

    (d) Sales to an out-of-city customer with shipments or deliveries to the customer's location within the city are considered sales made in the city.

    (e) Sales to an out-of-city customer with shipments to an out-of-city destination are considered out-of-city sales.

    (2) In the case of public utilities, or businesses furnishing transportation services, "gross revenue" for the purposes of this section may be measured by such means as operating revenues, vehicle miles, revenue miles, passenger miles, ton miles, tonnage, or such other method as shall reasonably measure the proportion of gross revenue obtained in the city by such business.

    (3) In case the business of the taxpayer involves substantial business activities other than sales of goods and services such other method or methods of allocation shall be employed as shall reasonably measure the proportion of gross revenue obtained in the city by such business.