very true, it is hard for most people to measure between "imaginary" centers, especially the bottom bracket. I just figured it would be helpful for crafty folks to convert bicycles with vertical dropouts. I was thinking about getting an old hardtail mountainbike for this very purpose...

Mostly, I'm just curious. The answer may also have some application in my everyday work, to an extent, especially if I can keep my solution in the most general form possible.