The relation between the actual FOV and the monitor distance is calculated, in this old video you can see it visualized:
This is Monitor Distance 100%, and depending in whether you use the horizontal or vertical FOV to do this, you get either MDH 100% or MDV 100%. The method will for many make the scopes feel too fast, since you need to match the movement to the edge of the monitor (side for MDH, top for MDV) to actually match the movement. Monitor Distance 0% however, will match the tracking speed, i.e. the rate at which you move your mouse to track a moving target.