I am fillet brazing a road frame with vertical dropouts and came up with the way of mittering chainstays, which I thought I'd share.

1.jpg

As you can see I've fastened a spare bottom bracket shell and a 10mm bolt to the table at the required axle to BB distance.
This still requires a lot of hand filing and constant checks but I could make the mitters pretty much identical and ended up with a straight wheel in the end.