It's actually not a trivial task at all to take a rectangle within a rectangle and scale it to a rectangle within another rectangle. Keyboard Maestro has code to reposition palettes when you resize a monitor and the code runs to dozens of lines to keep the relative position in the window.
If you make the assumption that the window is entirely on screen 1 this:
Set variable W1 to %WindowFrame%Front%
Set variable S1 to %ScreenVisible%Front%
Set variable S2 to % ScreenVisible%Back%
I was just looking for something like this and it's quite helpful. Is there a way to cycle through three displays? Using this macro will jump the window between two out of my three displays.