That makes sense, I've set the default FOV to 90 now.
Yes, and I almost have one ready actually. You can see a manual test of it here (this link will only work for you) where you select a game and a sensitivity, and whenever you change game the sensitivity will be calculated.
There are a few issues however. This example is using all default values (FOV, aspect ratio, multipliers, Windows Pointer Speed etc). This will make a lot of conversion invalid depending on what settings the user actually have. So ideally any conversion will need all the fields in the calculator (except monitor size) to be accurate, but then it gets very complicated.
So it's not easy to find a good and user-friendly solution to this.