Wrage to the dpi?

Can someone help me my problem is that I have a mouse of minimum 800 dpi I want to take over the sensitivity of a Youtubers but he plays with a 400 dpi does anyone know how much I need to reduce the ingame sensitivity so that it is the same?

Note it's about the game Fortnite

The sensitivity on the x-axis in the Youtuber is 0.188 and on the x-axis 0.149

Is not it slower or not? Then you almost need a XXL mouse pad if you want to turn / look.

And no, I would never do that! Change the settings of a YouTube or iwem change! Why and for what? It depends heavily on how fast the mouse has to be for itself and how big the monitor is etc.