This has been an issue in a lot of games for me, but let's take Deep Rock Galactic where the max FOV in game is 120 multiplier, corresponding to 82.5 vertical FOV, but with a mod can be set to 125 which more closely matches the 86 vertical I want. In the past I could enter any FOV even beyond the supported range of a game and get accurate results, but as of some time back it spits out this warning:
Honestly, the warning is good, but I still want to see the calculated results, just with a note next to it. This would also futureproof games for updates adding FOV options or increasing their range. At the very least please make this an option in "Advanced" mode