1. Convert desired vertical FoV to horizontal (90 vert > 59 hor)
2. Convert horizontal FoV into radians (this becomes the fovTop value)
3. Divide fovTop value by the last digit of your aspect ratio (for example,

1.03 / 9 = 0.11444444444444444444444444444444 - This is for 16:9 aspect ratio)

4. Take the resulting number without truncating any digits and then multiply it by the first number(s) of your monitor's aspect ratio (for example, 0.11444444444444444444444444444444 * 16 = 1.8311111111111111111111111111111)
5. Truncate the resulting number to 2 decimal places (Using the above number, the result would be 1.83)
6. You now have your fovTop and fovLeft values for your monitor's aspect ratio (16:9, 1.03top and 1.83 left = 59 horizontal FoV) and desired horizontal field of view.
7. Enter the calculated values into the game's config file for your player profile and save.

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