That being said, I think in the end you will really only find door closed/not closed of any real value.
How about a largish potentiometer on its side with a weighted pendulum attached to the shaft.If this was mounted in a box and attached to the door,in the right orientation, as the door moved the pendulum would turn the shaft of the pot and the resistence would change.it would be safe from dust and dirt and you could map the resistence to the door position.
Put a magnet on the little black thing that travels along the track.
Mount a 2x4 alongside the track. On it, at strategic points, mount reed switches. Very easy to move them to "fine tune" whatever "it can stop here" points you set up.
I know it's hardly in the spirit of this thread but.... do you really need to be able to set the position of the door to a 1% degree of accuracy? I can't imagine why you'd need anything more than fully closed, fully open and perhaps a mid point? this could be achieved very easily with limit switches.