Oh, don't go looking for the chart—you won't find it! Mercury and Venus station at most at 28° and 48° from the Sun, respectively, while the Thema Mundi has them at 30° and 60°. (Though I'll grant that I have no idea if the orbits of the inner planets have been stable for billions of years...)

Chris Brennan has an excellent discussion of the Thema Mundi, it's origin, and it's rationale, in Hellenistic Astrology, pp. 228–232. JMG, to your question, Brennan believes it to be philosophical or didactic rather than practical.