Except as provided below, when a ticket is purchased before the transportation begins or is reissued according to Rule 255 (Rerouting), the fare applicable to a circle trip via participating carriers, or partly via participating carriers and partly via other scheduled air carriers and National Air Taxi Conference members will be for the portion of carriage via one or more participating carriers as follows: the sum of 50% of the applicable round-trip fares for the respective sections, constructed from the point of origin via the route of travel that produces the lowest fare for the circle trip for class of service used.