If the thickest part of the star map above is a total of 2000 lys, and can be a little thinner. Our sun is very near the center line of the galactic disk in the Orion arm. If you go 900 lys in any direction normal to the axis of the arm, you are at the edge of the galaxy or in the gap between arms. The void must be one the gaps to the next arm either Perseus going away from the galaxy center or Carina going into the galaxy center. Either void looks like ~1000 lys wide. I think who wrote the story did a little technical research with real distances; a good job.