Before, we established that visibility is discrete. With update 13, however, there is now a number called "detection risk". What does this number mean? How does a lower value affect my visibility?
It appears that the "detection risk" is the level that the detection meter starts at the moment a guard or camera spots you. So if you have a detection risk of 75, the highest, you will be detected very shortly after a guard or camera spots you.
In this screenshot, my detection risk is 35. The screenshot was taken on the same frame that the camera first noticed me (taken as a clip from a video):
In order to successfully stealth, you're going to want that number to be reasonably low. Around 20 is usually good for me. The lower the better, though.