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@rw101
I recently had to widen a pair of rear dropouts but I could not find any resource to tell me how much to 'overset' the spacing so as to give the desired final width.
I used a 25cm threaded bolt with wing nuts and big washers to push the dropouts apart. I needed to go from 120mm to 130mm. Pushing the rear dropouts apart to 140mm and leaving for two hours resulted in no change - they just went back to 120mm. Pushing to 150mm and leaving for two hours gave 126mm. Pushing to 155mm and leaving for two hours gave 127mm final spacing. By then I had worked out the ratio so I pushed them apart to 170mm and that gave the desired 130mm final width.
Therefore it looks as if once you get beyond a critical amount of stretch (the amount where the bending exceeds the 'yield strength' of the steel I suppose), which in this case was around 20% of existing width, then the amount you push the stays apart results in a final increase in a ratio of around 5/1. Obviously there is going to be a limit - what as a photographer I would call the onset of reciprocity failure, aka breakage - but you can push them quite a lot. You want 10mm increase, you have to push them apart by 50mm. This was double-butted 531 by the way.