I did the arithmatic mean first, but it is just the sum scaled by a factor of 0.5. But since the central place hierarchy tends to be a log-linear relationship, the geometric mean felt like it was probably better.

And who knows ... one day in the distant past I may have actually read some literature that suggested that the geometric mean is the preferable measure in general for this situation ... I definitely knew a lot more abstract regional economics stuff a decade ago, even if I knew a lot less about the real world problems of fighting for better public transport.

I've been accused of being a Marxist, yet while Harpo's my favourite, it's Groucho I'm always quoting. Odd, that.