Conversion
In short: Fortnite sens times 0.9696. 6% becomes 6. For your own value, just edit the sentence below.
In the game you find the setting under Options → Controls → Mouse → Horizontal and Vertical Mouse Sensitivity.
The in-game slider runs from 1 to 100 in whole steps. Set horizontal and vertical sensitivity to the same value. The in-game field of view is vertical; in the converter enter the horizontal value (90 vertical is about 121 horizontal). The result is rounded to a whole number, so your mouse distance differs slightly.
Rainbow Six Siege turns 1.03 times further per mouse count than Fortnite. That is why the number gets smaller while your mouse distance stays the same.
Rainbow Six Siege shows 11.5° more field of view than Fortnite by default (91.49° against 80°). The picture looks a little wider and movement on screen a little slower.
In Rainbow Six Siege the slider only takes whole numbers. The result is rounded, so your mouse distance differs slightly.
Multiply your Fortnite sens by 0.9696. 6% in Fortnite becomes 6 in Rainbow Six Siege.
Options → Controls → Mouse → Horizontal and Vertical Mouse Sensitivity. The in-game slider runs from 1 to 100 in whole steps. Set horizontal and vertical sensitivity to the same value. The in-game field of view is vertical; in the converter enter the horizontal value (90 vertical is about 121 horizontal). The result is rounded to a whole number, so your mouse distance differs slightly.
Fortnite: Default 80°, check in the game whether you changed it. Rainbow Six Siege: Slider from 60 to 90, vertical (default 60, about 91° horizontal at 16:9). Match the field of view as closely as you can; then the converted sens feels most natural.
No. The values are for hipfire. Zoom and scope sensitivities are set separately in the game.
No. Keep your DPI and only change the sens in the game. That keeps your feel the same everywhere.