With my (limited) experience with fitting much wider tyres to a 70's roadster, I'd say use the ETRTO sizes. I don't know how precise those are exactly, but they are the actual size in mm's and it corresponds with your own measurements of the frame. I went for maximum width and ended up with 1-2 mm space between the front tyre and the fender stays, which is going to be a bit of a problem if the wheel gets out of true on a long ride. Plan B was to replace the fender stays with ones that go round the outside, and plan C to return the tyres and order a size narrower. But it fits because ETRO sizes were precise enough, at leat in this case.