Yes the calculation is difficult, but I'm not sure that it's very different to that of any other capital intensive industry. The major difference between airlines and other capital intensive industries (eg motor manufacturers, power producers etc.) is that airlines sell a very wide number of products to a huge number of consumers at fairly low dollar amounts. A power producer sells to, at most, a handful of customers (the power suppliers in a competitive market), a car manufacturer sells far fewer units at much higher prices. But the understanding of the different fixed costs, the different variable costs and the different marginal costs should be similar.