Consistent mouse sensitivity using your mousepad as a ruler
Matching mouse sensitivities between games is useful, but typical sensitivity calculators don’t include every game, have some insurmountable accuracy issues, and are often just annoying to deal with. You probably don’t want to break out a ruler every time you try a new game, but there is a shortcut: use your mousepad as the ruler.
My mousepad is 45cm wide and my mouse is 6cm wide, so a full sweep across the mousepad is 39cm of mouse movement. One of the first things I do when I first boot a new game is adjust mouse sensitivity such that one 39cm sweep causes 1.25 turns of rotation in-game, for 31.2 cm/360. Other convenient values would be 1 turn for 39 cm/360, 1.5 turns for 26 cm/360, 1.75 turns for 22.3 cm/360, and so on. Once you figure out how far a sweep is and your target sensitivity all you have to remember is how far one sweep should take you, and then it’s as quick as it gets to set up for a new game. The main limitations of this method are that it can’t target any arbitrary cm/360 value and working around game-specific ADS sensitivity problems is more complex.
Calculator
Why synchronize sensitivities?
It makes your aiming skills transfer between games better. It’s absolutely possible to be excellent at aiming in a wide variety of games all at different sensitivities, but it’ll generally take more time and effort to get to a given skill level that way.
There are some exceptions. For instance, Counter-Strike generally plays best with slower mouse sensitivities than other FPSes, so it may or may not be worth matching that one.
Even if you don’t keep everything matched, this is one of the quickest ways to get your mouse sensitivity to a precise known-good value when you’re booting up a new game.
Why not typical sensitivity calculators?
Sensitivity varies a bit for reasons beyond just mouse CPI and your in-game sensitivity settings, giving sensitivity calculators inherent accuracy problems. Aside from mouse CPI deviation, operating systems seem surprisingly bad at passing every input to games correctly, and when they fail it slows down your effective sensitivity by some unpredictable amount. Calculators have no way to guess at this, while my method handles it as well as is possible (and makes it easy to spot-check your sensitivity if you feel like it’s inconsistent).
Calculators also can’t realistically cover every game, and if they get close they’re unlikely to be carefully checked on more obscure titles. How much of a worry this is depends on the obscurity of the games you play.