Below are steps to setup custom Field of View FOV values for Arma 3.
Locate your [User].Arma3Profile inside %userprofile%\Documents\Arma 3 - Other Profiles\[User]
Edit values located next to fovTop= and fovLeft= ; these should be at the bottom of the config file.
FOV Values for the common 1920x1080 resolution screen:
Users looking for FOV values for other resolutions may refer to: http://hia3.com/tools/armafovcalculator-en
You may also try using Haych's method of an external tool that will edit the file for you:
@Twinkie