This isn't really the section for this, because this section is for Out of Character discussion of online role-playing. It needs to be in either the "Rules" or "Dark Millennium" (when we get it back) section really.
But to actually answer the question, it would have to be near the Tau Expansion zones. Tau aren't across much of the galaxy, and because Chaos is everywhere, that's the most feasible place for it.
... if of course you bother to say where the world is. 99% of the time, I don't bother saying where in the galaxy my worlds are.
I'll say that they're "out of the way", in some made up "major sector", a "lynchpin world to the region" - and that's all people need to know most of the time.
While the world's proximity to its neighbours may matter (for purposes of accessible resources, etc.), its position in the galaxy is almost invariably moot, and is more likely to be a problem than a help.