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%
Now we have to figure out what W2 will be.
W2.width = W1.width * S2.width/S1.width
W2.height = W1.height * S2.height/S1.height
W2.left = (W1.left - S1.left) * S2.width/S1.width + S2.left
W2.top = (W1.top - S1.top) * S2.height/S1.height + S2.top
Something like this:
