Measures Additive Over Some Dimensions

In the DWTk2Ed, figure 11.2 - Trip-level flight activity schema, two of the measures are Gross Segment Revenue and Segment Miles Flown. The grain of the table is part of a passenger itinerary. Clearly Miles refers to frequent flier miles, which would be additive, but what if I wanted to use them to determine the distance of a Trip, or the distance an Aircraft flies?

If I sum up the Revenue I get a reasonable answer for any dimension, but summing Miles will overstate how far the Aircraft has flown or the length of the complete Trip. If I first take the average or maximum Miles value for a given Origin-Departure Time combination, things should work. Are there techniques to handle this or are we really talking about a different grain - and different fact here?

It's a semi-additive measure. A universal solution would be to store them in a different fact table with the appropriate grain so they are fully additive. Or you can rely on the functionality (or lack thereof) of your query tool and supply adequate training for the end users.