Make sure to follow the instructions that appear when you start the game and run the DirectVR scan after entering the actual game world (ALT+L or from the vorpX menu).
After a successful scan the FOV is perfectly adjusted for your headset and there is no need to do anything manually.
The message about running the DirectVR scan (press ALT+L after entering the game world) that appears in the headset a few seconds after launching the game.
Once that scan is done successfully the FOV is perfectly right. Just make sure to enter the actual game before running the memory scan, it does not work while you are in the menu.