One thing that bothers me is that a chunk of the map is all that is missing. It would be better, originally, if the map were complete but the part BB8 is before someone Erased the planet from the archives. Much like Kalas recently did on Rebels.
If I had a map of Maryland but a chunk in the middle were missing, it would be a lot easier to figure out where someone is.
I suppose the answer to that would be that even a small section of a galaxy is still hundreds if not thousands of stars. Also, even if the Resistance knew exactly where Luke was in the missing section, without the map and his route it would be far too dangerous to try and chart a course through what is essentially uncharted space. As Han said, ‘Without precise calculations we could fly right through a star or bounce too close to a supernova, and that would end your trip real quick’.
Now a bigger question is whether the First Order had the full charted map, or if this was a section mysteriously kept blank by the Empire. Perhaps they just never got around to charting it? The questions compound.