Thanks for your quick response!
I took another look in a game at some trade routes and can't make heads or tails of this.
So you can see right away that it isn't the sum of populations, like the wiki says, so I don't know where that came from. Maybe outdated information there. However, I tried 3% of production, but that gives you 1.044, which doesn't match the mysterious .58. So then I tried 3% of income, which gives .663.
The trade route is labeled as a basic route, so it must be using .03 somehow, not .1 or .05. But I don't know where the numbers in the tooltip are coming from.
Edit: After digging a bit more, I see that 'raw production' as a value is not listed in the tooltips. The production shown is 'all construction.'
So I guess the question now is, can I change it to be based on gross income, or is it hard coded to production?