Skwuruhl explained here that it will match the movement to a certain distance on the monitor. Example, 75% match it to 75% distance of the monitor. It's good for matching how far you want your in scope flick to match the movement/distance. Remember that the distance start from the center of the screen. For game to other game conversion, other methods are better. Edit: It's better for hipfire the 360 conversiont method.
I red somewhere that 56.25 is better for games using horizontal fov like overwatch with a 1920x1080 resolution (1080/1920*100=56.25). For games with 4:3 fov like csgo, it's better 75% no matter what the resolution you're using. Might be placebo effect, but it felt like a better match. Not sure how the fov play a role in this, but 56.25 was for overwatch and 75% for csgo. edit: Using 1920x1080
For windows sensitivity, I didn't find a clear answer. With my personal experience of using 3/11, 4/11 and 6/11 over at least 5 years, I concluded that there is no difference for your muscle memory. After using one of them for a long time, I was really precise on 2d like windows and it was not 6/11.
Higher DPI is more smooth, but might capture more of your mistakes if too high. Higher windows sensitivity capture less mistake, but skip too much pixels when too high. Find the right balance between the two that work better for you.
Don't worry too much about it. I tried to match with windows as an experiment, for fun and to learn. I recommend not using sensitivity higher or lower than what 100% or 0% give as a sort of safe boundary. The difference might be too high to adjust on the go. Anyway we can't match 2d and 3d movement perfectly, it's only to one point on screen like math wizard wrote.
Edit: been away for a week and I didn't see they answered you better on the other thread oh well.