I haven't given it an absolute ton of thought, but I'm not sure why you are using the "constant 1000pixel diagonal" for both displays?
It's been a long day, and thinking too hard makes the head hurt at this point, but I upgraded from a 19" 5:4 to 22" 16:10WS, so that my new monitor is pretty much my old one, but with extra added onto the sides. I can then run the game at 4:3 letter boxed to simulate my old display, and also in 16:10 WS.
My initial instinct is that forcing a constant 1000pixel diagonal is akin to some measure of physical size (eg 19" 4:3 vs 19" WS), which I'm not sure is relevant, as it's really only aspect ratio and FOV that factor in?
It could just be me, but I'd definitely appreciate more insight into the reasoning behind your methods :)
Aggies
I chose to equate the sets based on diagonal dimension, but there are definitely other options. As another poster stated, the choice of 1000 was done just for easy math. It could have been any number, and any measure of length (e.g. 42 inches).
You need some metric to scale the screens. Otherwise, the calculation would have to consider the actual physical dimensions of the screen, and not just the ratio of width to height.
The problem is that there are a limited number of characteristics to describe a rectangle, none of which are perfect. You could choose to scale based on screed width, height, area, or diagonal. I chose diagonal because it is the de facto standard for measuring screen size. But again, you are correct. There are other options.