Identify the axis
Check whether the game exposes horizontal or vertical FOV. Some use a fixed reference aspect ratio, even when the actual viewport is wider.
Use the actual viewport
A 1920 by 1080 viewport has a 16:9 ratio. If the game is letterboxed or windowed, use the dimensions of the rendered viewport.
Convert and verify
Choose the direction and enter the source angle. For example 90° horizontal at 16:9 corresponds to about 58.72° vertical. Test the converted value in a cockpit or practice scene.