You are probably moving the wrong slider. The 3 things you need to be concerned about (Uncheck "Reserve edge of screen", Uncheck "Automatic distance from edge", and the "keep right on edge--keep far from edge" slider) are all circled in my screen shot below.
The "keep right on edge--keep far from edge" slider, which becomes active after you uncheck the "Automatic distance from edge" check box, is marked with the red arrow in my screen shot.
The computer I'm on at the moment only has the free version, hence the dialog box in the screeny; but the Plus version is very similar in concept.
