Select "Savefile" in the Location box for Fortnite:
This will give you a new more accurate sensitivity calculation, but you need to use a third-party program to edit the file. The link to the program is in the Game Info notes (also here).
Pay attention to this Location box in all games, it may include Config File, In-game, Console or Regedit.
I'm not quite sure why they went with this model, but it's quite easy to use at least. All the sensitivities do exactly the same math-wise, but is affected by the FOV of the different scopes. In other words, if you set all the sensitivities to the same value, you will have a 100% Monitor Match sensitivity.
CSGO is by far the most used baseline actually. You might have a different preference for 3rd person though. Even with everything equal (FOV and 360 distance) 1st and 3rd person might not match up to your liking, you might prefer one of them slower or faster.