A modern mouse nowadays can have a range of DPI from 200 to 8000 or even
more. The value can also be customized if the mouse has its own
customization program such as Logitech Gaming Software.
The formula for calculating new sensitivity:
Old sensitivity x Old DPI ÷ New DPI = New sensitivity
For example
Old sensitivity: 3
Old DPI: 400
New DPI: 800
3 x 400 ÷ 800 = 1.5
New in-game sensitivity = 1.5