I keep running across people whose stud rules have those three bases listed as opposing factors for each other, so that breeding two together yields the third, like:
Ennead x Ogdoad = Ra
etc.

But, the wiki has them listed as factors that are grouped on the same side. My interpretation is that breeding any of the three to a base that corresponds to one of them has a chance of yielding the corresponding one, like:

[Ennead or Ra or Ogdoad] x [Deshret or Nefer or Wepwawet] = Ra
[En/Ra/Og trio] x [any other Ennead factor] = Ennead

With the caveat that two of the trio bred together can yield either one (but not the third), like:

Ra x Ennead = [Ra or Ennead]

Am I right? Or am I throwing my beetles into the void breeding an Ubaste to a Ra in hopes of an Ogdoad?