Like how mouse sensitivity is subjective, the "optimal" DPI (Dots Per Inch) for Fortnite is also a matter of personal preference and play style. We at Game Gavel advise using a DPI of around 800. Our recommended mouse for Fortnite also has a DPI-shift button, allowing instantaneous adjustments to the mouse's sensitivity.
If you're unfamiliar with DPI and want to learn more, this helpful tutorial will point you in the right direction. In addition, consider the points made below.
The following formula reveals the true sensitivity:
Electronic DPI (eDPI) equals sensitivity (in-game) multiplied by Physical DPI.