ARMA 2 and ARMA 3 can already switch between a standard and "custom" aspect ratio without restarting the game or having to close and edit the config.
You're really only changing the field of view unless you use a wrong value, the aspect ratio is still the same if you did the calculations correctly.
Please see the attached screenshot for a concept of in-game FoV setting.
Desired end results:
1) No more converting between horizontal/vertical degrees and radians
2) Ability to set desired horizontal FoV within the game, without restarting to apply
3) User-friendly configuration