I found a copy of Hellenistic Astrology at a nearby used bookstore. (This struck me as lucky, but then I've had nearly a dozen "lucky" finds at the bookstores around here in as many days, so perhaps it's more accurate to simply say that the bookstores here are so much better than the ones in New York that it's ridiculous.)

I'm starting to read it now—it's introduction is lengthy, but I have a fascination with history so I expect it to be a while before I get into the meat of the practical sections—but I ran across this chart detailing the oktatropos which certainly feels like a relation to the scheme I'm working on, where the IX through XII are "derived houses" without an inherent meaning.

Thanks for the pointer!