Number-crunching : How many with class levels among ordinary folk?

I use a really simple formula to figure the demographics of classed characters among ordinary people:

• A man-at-arms (i.e. someone with just enough training to use armor and weapons) : 1 in 100
• A level 1 character: 1 in 200
• A level 2 character: 1 in 400
• A level 3 character: 1 in 800

etc.

Which means:
1) Characters with class levels are literally the 1%.

2) If you go by 1200s demographics (300 million people worldwide), there are 12,000 name-level-and-up characters worldwide (a single guy holds the record at level 17).
Which seems like a lot, obviously, but scale that back to, say, medieval France (approx 12 million people, largest European country by then) and that's 400 name-level-and-higher characters in the entire country. That sounds good to me.
If you assume 25% are fighters and 25% are magic-users, then there's about 100 lordly strongholds and 15 wizard towers in the country (taking into account the harsher tower-building requirements). 15 guys with the ability to sling 6th level spells in the whole country feels like it's just enough.
Something the size of Languedoc (1/25th of the country's area, approx. the area of the Isle of Dread for comparison) has 4 lordly strongholds and 0.5 wizard towers.
If you go with 1600s demographics you can just double the numbers above (barring the fact that France's population didn't double, plague and all). And have a guaranteed wizard tower I guess!

3) With all of this I can eyeball how many classed characters a location has.
Village with 300 people? One level 1 character, probably a knight and his squire. It took about 150 to fully support 1 trained dude in armor and all that; numbers check out.
A fort like Flint Castle (guarded by 120 men)? Let's say they're half ordinary folks, half men-at-arms or higher. That's 30 men-at-arms, 15 level 1s, 7 level 2s, 3 level 3s, two level 4s, and one level 5.
A large army or really big castle like Krak des Chevaliers or Kerak Castle would house hundreds if not thousands of classed characters and a half- or full dozen of name level PCs. In my opinion, that's alright. Those castles were involved in the Crusades, isn't that perfect high-level-play material?

4) I get to tell my players exactly how rare those hard-earned levels are. Might be cool to know you're a one-in-ten-thousand deal when you finally retire.

Sources:

https://web.archive.org/web/20060113004308/http://www.io.com/~sjohn/demog.htm
The article lists its own sources.