If fov's are similar or pretty much the same - use 360 conversion.
However, if fov's a different, like CS:GO and Fortnite use viewspeed. This is because if you match 360 distance the sensitivity from Fortnite would feel slower in CS:GO. This is due to CS:GO having a larger FOV than fortnite. So Viewspeed solves this by matching the crosshair speed to make them feel the same . It works in a similar way as this:
https://www.youtube.com/watch?v=RYCrGAJshvQ