Oct 1991, Clay Reed writes in an article titled “More Math for Progressive Thinkers.”:

Tertiaries:

May 1994, Clay Reed writes from an article titled “New Thoughts…Old Techniques.”To my knowledge, every available tertiary formula—in print or in software –is incorrect. The error is in equating “day” with “rotation”: the chart is progressed one civil day per lunar orbit, whereas it ought to be progressed one rotation per orbit. It’s astounding and unfortunate that this error has been perpetuated for so long. I encourage astrologers to lobby software companies to replace the faulty formula with the correct one.

Aug 1991 issue titled “Tuning the Well-Timed Chart.”…like every other primary method, takes forever and is based on faulty principles. Thus, I closed the book on primary directions; and, four years later, opened a new book on primary progressions. While working on my one-time American Astrology column “Astrology for the 90s,” it occurred to me that progressions formulas for primary may be valid only if they are based on strict, discrete, natural astronomical units. From this simple premise, I devised formulas for primary, secondary, and tertiary progressions. By an intellectual coincidence, my secondary formula (a single calculation) produced precisely the same result as Cyril Fagan’s (a tedious, page-long algorithm). Meanwhile, my primary and tertiary formulas were brand new—or, at least, never before recorded in the English annals of Astrology.

Jim, I will try to find chart examples from Clay’s writings to offer, but can you tell by what Clay has written above, according to Clay, is SF using a terts formula different from Clay's? I have asked Derek, given enough time, to offer his thoughts since he has most of the 90s AA issues.I’ve found that partile transits from outer planets to tertiary outer planets are powerful “triggers” for the exact timing of events.