But isn't this one example of doing it manually?
First I set my game I'm converting from to CS:GO with a sens of 1.685.
Then I set the game I want the sens to be converted to Fortnite (for this example).
I lloked up the sens it gives me for Monitor-Distance (33%) (0.010872) and V2 (0.011868). (A difference of 0.000996)
Now that the Monitor-Distance is lower I increased FOV so it get's higher.
New fov: 82
New Values: M-D (0.011244) and V2 (0.012214). (A difference of 0.00097)
New fov: 86
New Values: M-D (0.012015) and V2 (0.012916). (A difference of 0.000901)
Then a test if I can get the difference lower:
New fov: 85
New Values: M-D (0.011819) and V2 (0.012739). (A difference of 0.00092) (Get's higher)
New fov: 87
New Values: M-D (0.012215) and V2 (0.013094). (A difference of 0.000879) (Get's lower)
So...
New fov: 90
New Values: M-D (0.012829) and V2 (0.013632). (A difference of 0.000803) (Get's lower)
So we go higher
New fov: 92
New Values: M-D (0.013253) and V2 (0.013995). (A difference of 0.000742) (Get's lower)
I made a jump to see where the turning-point is
New fov: 98
New Values: M-D (0.014603) and V2 (0.015106). (A difference of 0.000503) (Get's lower)
Another jump
New fov: 110
New Values: M-D (0.017726) and V2 (0.017414). (A difference of 0.000312) (Get's lower)
New fov: 111
New Values: M-D (0.018017) and V2 (0.017611). (A difference of 0.000406) (Higher than with fov 110)
New fov: 105
New Values: M-D (0.016347) and V2 (0.016439). (A difference of 0.000092) (Get's lower)
New fov: 106
New Values: M-D (0.016613) and V2 (0.016633). (A difference of 0.00002) (Get's lower)
So in theory you have a more constant similiar distance to CS:GO with sens 1.685 when using an fov of 106 in Fortnite
(When I made this I was a little bit under time-pressure so please excuse any writing-errors or unnecessary fov's.)