Someone tells you their sensitivity is 2. This is roughly as informative as telling you their shoe size is 2 without saying which country they bought the shoes in.
Every engine converts mouse movement to rotation at its own rate. That rate, the degrees of turn per mouse count at sensitivity 1, is called the yaw value, and it varies enough between games that the same number can mean a two-fold difference in how far your hand travels.
What eDPI fixes and what it does not
The usual improvement is eDPI, your in-game sensitivity multiplied by your mouse DPI. It is a real step forward, because it accounts for the fact that 800 DPI at sensitivity 2 and 1600 DPI at sensitivity 1 feel identical. Within one game, eDPI is a complete description.
Across games it is not, because it still has no yaw term in it. Two players with the same eDPI in two different titles can have completely different aim. The eDPI calculator is the right tool for comparing within a game, and the wrong one for moving between them.
The number that travels
Centimetres per 360 is how far you physically move the mouse to turn all the way around. It is a distance on your desk, so it means the same thing everywhere, in every game, on every mouse.
cm/360 = 360 ÷ (yaw × sensitivity × DPI) × 2.54
Source engine titles use a yaw of 0.022, which is the most widely documented value and the default in the cm per 360 calculator. At 800 DPI, a sensitivity of 1 gives 51.95 cm for a full turn and a sensitivity of 2 gives 25.98, exactly half, because the relationship is a straight inverse.
To move between games, work out your cm/360 in the one you know, change the yaw to the new game, and adjust the sensitivity until the distance matches. Your muscle memory comes with you. Copy the sensitivity number instead and it does not.
Is there an ideal?
Not a single value, but there is a range. Most competitive players sit between roughly 20 and 50 cm for a full turn, which at 800 DPI and a yaw of 0.022 means a sensitivity somewhere between about 1 and 2.6.
Lower than that and you cannot turn around without picking the mouse up. Higher and small corrections become difficult, because a millimetre of hand movement covers several degrees. Within the range it is a genuine preference: lower suits deliberate aim on a large pad, higher suits tracking and fast turns.
The practical constraint is often the desk. If your cm/360 is 40 and your mousepad is 35 cm wide, you physically cannot complete a turn in one motion, which is the real reason most players settle where they do.
One thing to turn off
Mouse acceleration makes all of this meaningless, because the distance then depends on how fast you moved rather than how far. If you want cm/360 to be a property of your setup rather than a property of each individual flick, turn acceleration off in Windows and in the game. For the other half of aim consistency, see the piece on frame time.