Sorry to flood your post, but I just thought of an even easier way to setup a control bone. Instead of using offset(so uncheck that on the follow path constraint) use the time evaluation. By default the evaluation is 0 to 100. Click the curve/path then under curve/data tab of the properties editor under the path panel change time from 100 to 359. That way it goes from 0(start of curve) to 359(end of curve). Then in your driver you can use:
degrees(var)%360
Where var would be the axis(X, Y, or Z) rotation of the control/driving bone.
