Part 1)
I'm sure there will be disagreement here, but if you go in front of a doorway with a circle on it, you can see that the horizontal and vertical sliders MUST MATCH to maintain proper aspect ratio.
If there's too much disagreement on this, I'll post screenshots tomorrow of 4:3 vs 16:9 and various slider settings when close to an in-game circle.
This premise is what guides all the calculations below.
If they don't match, you are throwing off the relationship between horizonal FOV and vertical FOV. Now, if the devs did widescreen properly, you would NEED to adjust them differantly. They didn't, they essentially zoomed in (and cropped, but that part doesn't really matter for this). So you are essentially viewing a 4:3 screen on a 16:9 monitor for example. The devs are maintaining the proper 4:3 relationship between vertical and horizontal FOV already. So you merely have to zoom it back out, keeping the same relationship between the vert/horiz FOV's/sliders.
Essentially all you need to do to fix the devs mistake is zoom out until the heights match again without the devs cropping (i.e. compensate for the dev's cropping). The sides will take care of themselves.
Part 2)
Here's the numbers for BOTH sliders if you want to keep the base FOV of 75 degrees that the devs use
Table 1:
16:9 = 0.75 effective FOV = 100 degrees
16:10 = 0.833 effective FOV = 90 degrees
5:4 = 1.066 effective FOV = 70.4 degrees
4:3 = 1.0 effective FOV = 75 degrees
Those are the corrective numbers to fix the zoom in that the devs do. That will reverse the zoom back down to the point where it will add the proper FOV to make up the differance between widescreen and full screen.
In the case of 5:4, its more square, not more widescreen. I dont have a 5:4 screen so I cannot verify that the game does a reverse zoom on it, but since it seems like they want to lock it to a FOV of 75 degrees, thats what makes sense to me. The only way to tell would be to switch resolutions between a 5:4 res and a 4:3 res and see if you gain part of top and bottom, instead of losing it on screenshots
[The numbers are derived by looking at what it takes to get the heights on a ratio to equal. So for instance, using a factor of 4 (this number is something to do with LCM least common multiple I belive, I know how to do it but I'm not sure of the exact term), you can multiply a 4:3 ratio into 16:12 ratio without changing the ratio itself. Now you have equal widths, 16:12 and 16:9. So you look at the height differance, its a factor of 0.75 (9 divided by 12). So theres your 0.75 corrective factor for thier zoom. Find the LCM between 16:10 & 4:3 and the LCM between 5:4 & 4:3 if you wish to verify the other numbers]
Part 3)
This part is if you want to start with a base of 90 degrees (for those who want to just increase the base FOV without regard to the devs design of 75 degree FOV). FOV of 90 degrees is found in such games as Doom, Half life, Quake 3, UT2k4
I used the FOV calculator here, telling it I had a base of 90 degrees:
http://www.widescreengamingforum.com/fovcalc.php
And then took those numbers to get an idea of what the slider should be
Table2:
16:9 = 0.706 effective FOV 106 degrees
16:10 = 0.75 effective FOV 100 degrees
5:4 = 0.869 effective FOV 86.3 degrees
4:3 = 0.833 effective FOV 90 degrees
Part 4)
If you want an exact FOV, just divide 75 by your desired FOV. For instance, if you want 105 degrees FOV, 75 / 105 = 0.714 for both sliders.
The reason this works is that the devs designed the game to have a constant FOV of 75 degrees, regardless of your resolution, which is where thier unwanted zoom comes in the picture.
HUGE thanks to Racer_S who did the tool!
Also thanks to Drexion who has a 5:4 monitor and helped me out with some stuff.