This number is surprisingly well known in all the books I've read, although I'm going to pre-apologize that I don't know "how" they come up with the number, whether it be from census data or archaeology.

The average throughout the empire was about 80-90% rural farmers, with the lowest percentage being Egypt, which was only about 66% farmers. Which would seem to indicate that areas of greater urban density, like Greece, Anatolia and the Levant would tend toward the 80%, and areas of lower urban density like Spain or Britain closer to the 90%.