good info, as always, Dr. Farr.
My understanding is that the decans originated in ancient Egypt. They were originally based upon the heliacal risings of particular stars that appeared at 10-day intervals. Which sort of works, if we consider decans today as 3 10-degree sectors of a 30-degree sign or 30-day month. (The Egyptians added 5 inter-calculated days to their calendar.)
You are aware, but perhaps not some of the others, that a number of ancient societies used the night sky as a giant outdoor calendar, and they tracked the risings and settings of particular fixed stars and asterisms to determine dates for agriculture and religious festivals.
I think Egyptologist/astrologer Joanne Conman talks about the ancient Egyptian decans in some of her articles:
http://independent.academia.edu/JoanneConman
Firmicus Maternus (4th century CE,
Mathetheos) seemed to have some familiarity with the Egyptian system, but implied that it was based in religious secrets, so he couldn't say too much about the decans. I believe I read somewhere that the decans were the basis of the Egyptians' celebrated medical knowledge.
I'm traveling now and don't have my copy of the Bram translation with me, but maybe somebody else here has it and can say more.
http://www.hellenisticastrology.com/astrologers/firmicus-maternus/
http://www.skyscript.co.uk/firmicus.html