Using 6/11 windows sensitivity is best in precision games. The last thing you want is that you can not make a shot because your mouse simply skipped too many pixels, which is what can happen if you have it set too high. If you have it too low on the other hand, other annoying things can happen. Google it.
You can just use a decent base sensitivity that is somewhere close to the sensitivity that you like in games. It does not have to be perfect. Settling for 400, 500, or 800 DPi is fine since you can simply change the sensitivity in-game to fine-tune it to your exact preference. For example , in Counter Strike GO I use 500 DPI, Windows mouse setting 6/11, ingame sensitivity 1.75. Among pro players, settings of 400-600 DPI with in-game sensitivities ranging from 1.5 to 3 are very common, giving them a fairly low sensitivity compared to most players. Both current (f0rest) and old (Spawn) top competitors had settings like these. They practically all use windows settings 6/11 as well though with some exceptions. As always , it remains more about skill than hardware or settings.
In response to the comment that ambidextrous mice can not be comfortable. Ambidextrous mice can be quite comfortable, and there are tons of people who prefer this classic no frills shape over any other, myself included. Even among the top in competitive gaming in both RTS and FPS it is still the most popular shape. With mice with shapes like the Xai, Kana, intellimouse, wheelmouse optical, AM and FK still being the most common.