Using just the wheel and frame, I determined my DS dropout was 2-3mm more outboard than the NDS one... ally frame, so no tweaking. At least it was in a favourable direction, so I dished my wheel to match the frame.
Neither dit it sit squarely in the frame, so some filing of the dropouts was also necessary... all done by eye, so it's not perfect, but it's a damn sight better than it was.
...Some of you may be dubious about that, but I assure you, I have my head around all the relevant considerations, and many pains were taken. Takes quite a bit of thought to ascertain what's going on just by dishing and flipping...