Curtis: Thanks for posting. Mystery solved!
I've never seen (or heard of, for that matter) an framing square with octogon rafter tables and it never occurred to me that the measurement for the side cut for the Jack Rafter would be given in terms of Eave Length/16.
In terms of multiple decimal points:
6 × 12 + 7.25 = 79.25"
16 × 12 = 192"
arctan(79.25/192) = 22.42889741° ... O.K., that's close enough, we're not doing a moon shot.