The Crossley mounting often makes it difficult to tell if the
telescope is being occulted by the edge of the dome slit. The safest
way to assure yourself all is well is to go down and sight up the
telescope tube from the bottom. If you do this a few times you will
soon acquire confidence in your sense of where the telescope is
pointing as seen from the top. Another way to tell is to sight out of
the slit along an edge of one of the junction boxes at the top of the
telescope, if that edge is itself parallel to the telescope tube.
Since the telescope drives east to west, sight along a parallel edge
on the east side of the tube and put the east side of the dome slit
close to the tube. This gives you maximum clear slit to drive toward
in case you get busy and forget to recenter the dome.