Hi,
please excuse my short answer last week. I was browsing the forums on my phone, not really comfortable for long texts, even posting a link is a hassle.
The Answer is: There is no such thing as a correct FoV.
The Field of View is fixed by the game developer. If you wan't to know how to define the Field of View in a Game best ask a Game Developer.
I don't know any mathematical way to correctly calculate the Fiel of View, as in every caluclation you have the radius (distance from eye to the monitor) and this is not a constant Value, some user sit closer, others further away from their Monitors.
That said most Shooter games use a hFoV of roughly 90° on the PC and around 60° on the consoles, because a TV is way further ahead than a PC monitor.
For your specific question with the 21:9 Aspect Ratio, if you use the FoVCalc, input a 16:9 Ratio with your given 82.5° hFoV and your target 2560x1080 Resolution you get a new hFoV of about 98°
greetings
Haldi