Lazza
1. How can a definite angle exist for a given screen size/ratio, game-FOV, and user-distance to screen? In IR, AC, and (I think) LFS you can change the rendering on the outer monitors only - which therefore allows almost any angle you desire - without changing either the screen size/ratio, game-FOV, or user-distance to screen. Therefore, depending on how the outer monitors are rendered, the outer monitor angles can almost be limitless (or at least limitless between 0 degrees and 90 degrees)...
Unless I'm mistaken, you've just pointed out exactly where rF2 is lacking. In rF2 the side screen angle is determined by the things you've listed, at some point it should be able to cater for your current screen layout. At the moment it very much limits you to a single correct side screen angle, which will be the same as each screen's h-FOV (note the h-horizontal).
What gets drawn in the screens doesn't overlap or leave gaps (otherwise you might manage to fake bezel correction!). So if the centre screen has a h-FOV of 80°, the edge of it is showing you what's at 40° away from centre, and the next screen will start drawing what's above 40° from centre (of the centre screen), across to 80° from centre at the middle of that screen (120° at the far edge). The centre of that screen has to be pointing directly toward you for it to look right, and since it's showing what's 80° off-centre it obviously has to be at a 80° angle, and will be 80° to your side. Then it all works.
When they allow you to specify your screen size and angles, the game will instead adjust the multiview angles to suit your layout and your screens become windows into the game world exactly where they are, instead of needing to be in the correct position for everything to look right.
So back to your question, the angles are currently determined by the h-FOV, which depends on the screen ratio and v-FOV (not directly; horizontal:vertical FOV isn't the same as the screen ratio).