This is the latest maps I have of the Galaxy, taken from Dark Millennium, which came with the current rule book.
If each of the small squares represents a Sector, then I would suggest that the whole of the Tau Empire is within the Lithesh Sector. It would then make sense that we are to the southern end of the sector as shown, so that our craft are able to reach the Zeist Sector, which would either be the square directly below, or the one to the south west.
Also found this little gem, taken from 1st Damocles.
"After a 5-month-long journey across the Damocles Gulf, the Imperial Crusade fleet arrived in the Dal'yth System within the Kendral Sub-sector. The outermost planet in the system, Pra'yen."
This would hint that the Damocles Gulf extends further than most maps show, even as far as Ksi'Myen System.