mr_jrt - ah, but the airport not the council are paying for the 200mn rail link, hence airport parkway not luton town station. This is the same reason why it won’t connect to the town centre instead - tourists/business trying to get flights won’t want to go to the main centre and most of the (passable) hotels are clustered around the airport.
I would imagine EMT customers would prefer stops at the airport rather than town - some for flights; others simply because it’s a better station.
For distance measuring, I thought Marble Arch for the roads? For trains, as someone else mentioned - zone 1 often suffices. Or more practically, the first connection to the circle line - hence St Pancras from the north, and blackfriars from the south.