I'm used to have 90 as the vertical FOV in games, and since the Source engine is kinda whack I've also used this site to get which FOV I should have by trying numbers until I got to the closest one. Which in this case was 106.2602.
Today I tried to calculate the FOV by moving 45 degrees (((360distance / 360) * 90)/2) to where the top of the camera was in Titanfall 2, a game made with Source. After that I also tried it in my Unity project, and it was different, despite the 360 distance being pretty much the same. After that I went into Minecraft to test it, where I'm sure the FOV is 90, and the Unity project was much more accurate.
After that I tried 90 * (90/73.739795), which results in 109.8457081 as the ingame-FOV in Titanfall 2.
This is a lot more accurate than before, and makes more mathematical sense, but just in case I'm stupid you might want to test it out.