The way I try to think of it is:
When it comes to mouse sensitivity, there are two factors at work:
Muscle memory, and hand-eye coordination. If you change the in-game numbers, they alter the muscle memory, if you change FOV, it alters the hand-eye coordination. If you increase the FOV, you need to alter the muscle memory a little in order to compensate if that makes sense. Like if you have 3 * 4 = 12. And you change the 3 to a 2. It needs to become 2 * 6 = 12 for the equation to work. You need to alter things proportionately to eachother.
The calculator does this for you which is why you end up with a smaller 360 distance (changed muscle memory) due to the FOV being different (Changed hand-eye coordination).
If you go in-game and put your FOV down to 60 and do a few T-hunts, then change it to 90 and do a few, You'll notice that it feels slower on the 90 setting. That's the hand-eye coordination fucking up because the FOV got changed.
That's the way I explain it. Essentially just do what the calculator says and you'll benefit. Trust me. I play Siege, BF4, CSGO, Fortnite and recently Overwatch. And using Monitor Match at 0% I maintain my aim through all the games, despite them being different 360 distances doe to the difference in FOV.
If you want the same 360 distance, you definitely wanna match FOVs. Specifically, you wanna match Horizontal FOV. You see that "Actual HFOV =" number. Reduce the 21:9 FOV until that HFOV matches, or is as close to the same as the standard 16:9 one. That can also give you the same sensitivity. It'll probably need be between 65 and 75 I would guess.