Originally Posted by bst
Just use this:
1) Put in your details in the top section ("Settings").
2) Press calculate next to "real sensitivity (inches/360)", under the "Sensitivity" section
3) Press calculate next to "estimated useful dpi", under the "DPI" section
4) If the DPI in the box next to "estimated useful DPI" is the same as your DPI in the top section, then its basically 1:1, if its higher then its beyond 1:1, if its lower then its below 1:1.
5) Change your sensitivity in the settings section until you see the same DPI in the settings section as the DPI in the "DPI" section (where it says "estimated useful DPI".
6) Now you can change your DPI in the settings section, and you will see that it always matches with the DPI section. So the sensitivity that you've set in the settings section is the one that you want for 1:1, and you can then adjust your DPI in order to attain your desired sensitivity, instead of using the sensitivity option in game. It will change depending on your FOV and screen resolution, by the way.
You don't need to have 1:1 btw, just make sure you don't go over 1:1 (eg 1:2), below 1:1 (eg 1:0.5) is fine. Keeping that in mind, and using the in-game sensitivity option, gives you the most choices. For example if your mouse can only select between 3 dpi steps, then having 1:1 limits you to 3 vastly different sensitivities, which you would be lucky to find comfortable. Also, 1:1 is kind of unattainable in the real world with FPS games, since FPS uses angles, you can only get close with a calculation (afaik, anyway).