I've noticed that they often just straight up switch values with each other, is this intended?
E.g. Fidre village has -2 demand for preserved fish, Näret village has -1 for it. They set up a trade route. They now have just reversed roles.
Not really, no. You can leave them as-is. The point of trade routes is just to bring the values closer together to avoid absurd arbitrage opportunities for nearby settlements.
this only happens if they differ by exactly one. if the demand modifiers were -3 and -1, they'd both move to -2.
The easiest thing to do is to just ignore differences of 1. The second easiest is, when there's a difference of 1, pick the more extreme of the two values.
Another option would be to interpret it as "moves UP TO 1 step, stopping at equal". in this case, you'd be switching to -1 and 1/2 for both, in which case you'd drop the fraction and have both at -1. Not sure if that makes sense, since it represents two cities with surplus having less or two cities with demand having less, but it's an option.
Yeah... interestingly enough, moving up to one step and dropping the fraction will lead to the opposite of the most extreme of the two values in every case, since dropping the fractions is always a move toward zero.
I'll just leave them both unchanged if the difference is exactly one. The mod isn't a super big impact on the price anyway (b/c 4d4, an extra +1 is typically 1.5% higher price -- about 15-17% chance of 10 extra percentage points).
Thanks for the advice!
Phew!
And now I've recorded trade modifiers for everything on my local map (Map Style IV, 120×125 miles) -- 31 goods, 94 markets -- and have set up a script for me to be able to find trade modifiers for a market on the kingdom map if needed -- the terrain and type-of-being (dwarven/elven) is recorded for the kingdom map, but trade routes aren't yet.