For ordinary desktop work I keep my mouse at 3000dpi. I have 3 monitors and like to be able to move the pointer across all three with just a couple centimeters of movement, so 3000dpi does the job. Usually when I tell someone about that they think I couldn't have any amount of precision at that sensitivity, but I've been doing this for a couple of years and can easily nagivate around spreadsheets, menus and web forms a that sensitivity.
For most types of games I also leave it at 3000, however for online FPS switch it up a bit. For sniping I lower it down to 2000. For highly aggressive close range play with a kinfe or shotgun I turn it up to 4500. For standard run and gun style tactics I just stick with 3000.
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