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.