The ring doesn't change the focus on the lens. As I understand it, it changes how the lens corrects for aberrations and/or field curvature.
Usually I don't bother to read the distance off the camera side scale. I just estimate the distance and set it.
The detents are there because if you are working quickly, you can just set the ring to either the close detent, or the far detent (whichever is applicable to your subject) and the floating element correction will be very close to being optimum.