I'm sure there's a sound geometrical answer to this (astral navigation has never been my strongest point

so thank you, Universal Cartographics), but why do you need 6 points of reference?
Surely, if you have 3 reference points and the target's relative (galactic or local) angles of declination, etc., from each of those points, you can still perform the triangulation?
Even if the 3 points happened to be on a flat plane, the lines would still intersect somewhere - you'd only need a 4th (different 3rd) point if 2 of the original 3 were aligned to the destination.
This 6 locations theory seems (to me) to be based on the Stargate SG-1 way of determining where each Stargate is.