You already posted a lot of details of how pathfinding was calculated so maybe you could do this too ? You don't have to bother explaining everything, just post it and we will figure it out, or maybe its already in a game file and you could then tell us in which one it is (I already saw that some stuff useful only server side is nonetheless in the client so it's possible) ?
There is one case where it would be extremely useful: say I absolutely want to have boats thus towns near river/sea and for this I need to settle towns not exactly as close from each other as possible, then I wonder: with 4 towns, should I switch to monarchy or not ? That is, will I have more or less than 20% corruption in each town overall ? And if monarchy gives less corruption, then: will I have very bad corruption by making a new town ? That last one is not a big problem, if it turns out to be a bad idea I can just destroy the town and only a single settler is wasted, but if switching to monarchy was bad initially, switching back to collectivism/despotism have a huge penalty so we really have to know before please.
In fact, the best would be to have a "monarchy simulator" just like you made a battle simulator, in python or excel or whatever you want, it would be so much fun just experiencing with it even trying things that would be very hard/impossible to do in reality because of the mountains/forests in the way, or dumb things like looking how it would look with 10 towns in a straight line, 20 towns in 5x4, 9 towns in 3x3 plus one extremely far (like if it had been converted).
Please do it, because it's really not as simple as in freeciv where it simply depend of the number of cells between one city and the capital, here it's clearly the distance between each town from each other and even the one in the center doesn't always have 0% corruption.