The general consensus is to use Monitor Distance 75% if you have a different FOV (Field of View) between games. Unfortunately, most games do have different FOVS; if they have the same FOV, 360 distance matches perfectly and there is no need for monitor distance. The new Viewspeed Test (in the dropdown menu when you hover over calculator) should be the best for matching different FOVs unless you experience a bug.
There is no "perfect sensitivity" but generally between 25cm/360 to 52cm/360 is ideal. Higher than 52cm starts to be too slow for tracking and below 25 tends to be too fast for pinpoint accuracy. There are outliers to everything, of course. For instance, if you're sniping, you could use a really low sensitivity and be very effective, or if you're playing something like Quake, where movement is extremely important, something below 25cm could be used.
Edit: I should note that the range above is a guideline for 106.26 HFOV. If you have a really low FOV it probably would feel comfortable to aim at a lower sensitivity than usual due to your point of view. Conversely, at high FOV aiming faster than usual would/should feel better.
The purpose of this site in my opinion as someone who has used it for years is to find a sensitivity that is good for almost any game. This would be something in the range I said above and in this way, you can have similar aim no matter the FOV and game you play. If you choose a high sensitivity, you will be able to match it to more games in general (many pc games don't have the option for lower sensitivities, unfortunately). There is a trade-off to high and low sensitivities and a trade off to medium sensitivities as well.