I think many don't even look at CPM. They just assume that business/first class longhaul international flights are usually pricey, they just assume that a "saver" redemption at a decent rate is a good value. Then there's others who simply look at the price of the flight, and "instinctively" decide whether it's worth paying cash for it or whether they'll try to use miles.


In fact, how do you calculate the cash cost of the flight to evaluate CPM? The cash cost of the flight can vary a lot over time, while in most programs the "saver" amount of miles needed is fixed whether you redeem 11 months out or 1 day out. But "saver" awards are not necessarily available at the same time that the best fares are available. So do you compare just against the fare that was in effect on the day you booked the award, or against the lowest fare that was ever available from whenever the flight first was made available for sale (typically 11 to 12 months out) up through the time it went to airport control?

I thus posit that the CPM value for a particular redemption is actually a range, not a single value (because the lowest price charged for that seat in cash covers a range). If someone treats the CPM as "absolute", the paradox is that two people redeeming the same amount of miles for the same flights for the same date but doing the redemption at a different time can come up with a different CPM for the exact same "seat", simply because they used the "blind men and the elephant" means of looking only at the cost of the flight at the moment of award booking.


Meanwhile, I'm confused about your use of "one" ("when one should use miles"). Are you asking when you yourself should use miles (that's much easier to answer), or when any abstract person should use miles (that's nearly impossible to answer)?