I figured this out recently, but then discovered that Philon knew much of it over 9 years ago (as shown in reply #2 of Workers-Pupils-Students) and Trium knew the rest.

Assuming that a figure in [] is rounded down (if necessary) to an integer, the work force =
[[0.6 * (people aged 20 through 49)] * [percent of plebeians] * 0.01]

The same formula holds for Pharaoh (substituting non-scribes for plebeians).

Since people aged 14 through 20 can go to an academy, people aged 20 can both attend an academy and work!