The rail companies have distance figures and the fares could be

So which of the 5 routes from Dartford to London would you use to
calculate the fare?

The average would be the best way. Whatever it is, the variation is going
to be quite small, compared to the large variation the zones have
introduced.

Weekends, Cockfosters to Heathrow is £2 for a 49km journey. Kings X is also
£2 but you only get 16km. Should KX not be 65p? Or should Heathrow be £6?

Regards,
Dave