Inspired by the thread re NYC-West Coast fares ...

What seems to be the most accepted average CPM calculation, a simple sum of the CPM-per-trip divided by number of trips or a weighted CPM-per-trip multiplied by trip miles divided by total miles travelled?

I've always thought in terms of the former (due how I've set up a miles/cost tracking spreadsheet), but the more I think about it, the more invalid it seems to say that the average CPM arrived to via the former method is, well, valid.

Take, for example, 20 LAX-SFO-LAX roundtrips, and say each one cost $500. Then have 8 10,000 mile roundtrips for $500 each. The first method would give us 37 CPM while the second would give us 14 CPM (which, if you will note, gives us a mathematically valid answer... average CPM times miles flown = total expenditures).

Thoughts?