I have to be honest...Other than respecting the obvious thought you put into the article, this might be one of the most boring submissions I have ever read.
Here's an easier system. Make up whatever demographic you feel like you need off the top of your head when necessary. Any GM who simply knows his or her world, can use some common sense when it comes to figuring out how many wizards are in yonder village and what level they are.
Dont go by me...I'm sure there are people here who will love the math, charts, and figures, but for me...eeek! Go to Comment
To be honest, I was never a big fan of demographics and largely ignored them for a long time. Certainly they will never be allowed to obstruct a storyline in my campaign. If I need a high level character in a tiny village, I'll place one there.
The"Medieval demographics made easy" article linked to above, however changed that, as I found that dablling with the numbers opened up a multitude of considerations I hadn't paid attention to before.
My "four maxims for world bulding" article (http://www.strolen.com/content.php?node=2606) stresses among others, making connections in relation to aspects of it.
While the numbers in themselves are not neccesarily useful, it does lead to some interesting considerations for a setting: How many people actually receive training to a degree they qualify as one what you'd call the adventurer classes (personally, I think 'adventurer' is a very arbitrary concept. If anyone describes themselves as such in my game, people will perceive them as dilettantes).
A martial society will probably have more trained warriors than your average kingdom. Personally I think when you look at ancient Sparta, you have a society that is such a thourough concept, it would fit right into a fantasy setting as a 'fantastic' but still very neat society. Everyone's a trained warrior? ok... It also impacts considerations about magic guilds. It gives you an estimate of how many wizards there might be active in a given kingdom. Sufficient for large guilds? if not and you have one, it is worth considering what special conditions there are that makes magic so accesible to the common people.
Hvaing such a framework also gives you some idea of just what it means for characters (or NPCs) to progress to the higher levels. Can a character really progress to the 18th level and still be generally unknown? No. On the contrary, they likely become veritable celebrities, at least in the areas they have frequented the most, rulers should be offering them lands and titles as the common people will want such distinguished people to govern them far more than your average nobleman. They will be feeling the pressure of taking up responsibility in society; wizards will receive petitions to take up seats as head of mage guilds, fighters to accept leading positions in the armies and so forth.
In the same way, NPCs should follow the same pattern. Contrast it to Forgotten Realms, where you can have random 20+ level wizards inhabiting cities and towns that don't really impact much more than their local community. Such people should be known far and wide in countries far from where they live. Fame and high standing is price to pay for power and should be a difficult bullet to dodge.
Of course all of this is fairly superfluous if you don't demonstrate them in your game. Personally, I make sure that my players know where they came from and if they have any responsibilities concerning family, etc. A wizard coming from common ranks will, in a setting where they are not that common, probably be brought up with a sense of expentancy and responsibility. the one the family have vested their hopes and finances in. Go to Comment
The problem is, only a few people will actually read them and comment on them which is a shame and hence nearly not worth it. Look at al lthe articles we have placed up since v2.0, comment/ read ratio vs a normal submisison of another kind if not even. Go to Comment
I would add a fifth one which I've heard somewhere: For every setting element you create, also create some secret element behind it or related to it. This gives the PCs something to constantly discover or investigate. Some of the secrets might be earth-shattering ("the gods are really just powerful mortals?") but many will be minor ("ah, so the mayor has been in league with the bandits all along..."). Don't overdo it: if you can't think of a good secret, don't jam a lame one in there just to meet the quota; the point of this guideline is to create a setting with a lot of subtlety and mystery, not turn the campaign into an ongoing Scooby Doo episode. (As a point of procedure, I think it's easier to write down this secret knowledge in a separate place, so that you can show your "setting guide" to interested players without having them learn all the secrets.)
(BTW, I got here through Johnn Four's Role-Playing Tips Weekly, so hopefully some noob GMs did get exposure to these helpful maxims.)