Remember that the length of the actual chanistay is just part of the total chainstay length...it's C-C, so once you factor in the BB shell and the dropout, your mitered 425mm stay might yield you a 440mm actual stay length.

I'm designing a 26" wheeled commuter for my wife right now, and I think the BB drop is 55mm. She'll be running 170mm cranks with quill pedals, but isn't the "pedal through corners" type, otherwise I'd probably bump BB drop to 48-50mm.

No idea on the Surly stuff, I build my own forks.