Start by looking at the recommended hardware on the TSW Steam page (any route) and then experiment with the settings menu.
Set everything to medium/high, run a scenario or two. If all works smoothly and you are happy with the performance, up a setting or two and try again. Rinse and repeat until performance is less than you feel you can put up with. Then back off a step or so. Don't up too many settings at a time, or you won't be able to tell which ones pushed you over the line. Be aware of, though, that different routes can fluctuate rather wildly as far as how performance light/heavy they are.
