This paper explores an analytic, geometric approach to finding optimal routes for commercial formation flight. A weighted extension of the classical Fermat point problem is used to develop a scalable methodology for the formation routing problem, enabling quick calculation of formation costs. This rapid evaluation allows the large-scale fleet assignment problem to be solved via a mixed integer linear program in reasonable time. Weighting schemes for aircraft performance characteristics are first introduced and then extended to allow for differential rates of fuel burn. Finally, a case study for 210 transatlantic flight routes is presented, with results showing possible average fuel-burn savings against solo flight of around 8.7% for formations of two and 13.1% for formations of up to three.