I'm more than happy to help. Here are some thoughts I've got thus far:
- Death: So perhaps 1 in 10-15 mages die/go insane as babies/when their talent manifests/etc. This would be high enough to be a serious concern and allow you to increase the commonality of mages by an order of magnitude. This would probably have a fairly well-defined time span (modified by exposure to magic and if the mage has help or is discovering magic solo), after which it is generally assumed that mages have enough control/maturity to not kill themselves or go nuts. Mages that go insane from other things (mundane to harrowing experiences) do not fall into this category.
- Quitting: Given that magic is tied directly to the personality of mages, I would assume that early death and madness is a far higher likelihood than cutting themselves off from their magic, for your universe in particular. I would think the most rare of reasons. Unlike the Dresden'verse where magic is more accessible (think of Charity) magic in Verus'verse is much more personal - what you're capable of supports and reinforces who you are. Quitting that wholesale is probably a very unusual event. In fact it may represent such a personality shift that their magical abilities shift to reinforce their desire to be cut off from magic, say by dampening other magics.
- Quiet Lives: I would guess that most mages fall into this category. Most just want to live their lives, not get mixed up in international magic politics or devote themselves to magic enough to become prominent experts. Instead they likely live like normal people, using their gifts to augment their normal living. From the neighborhood mom who works as a nurse to the highly successful pimp downtown, they live like pretty normal people, just exceptional. These are bread and butter mages, not who a story would focus on. I would estimate that 50-90% of mages fall into this to some degree.
Adepts are, for the most part, not going to suffer these effects, as their extreme focus should seem very normal to them as it only reinforces who they are (and has likely helped define them). So to keep a logical balance between adepts and mages,
adepts should likely be about two orders of magnitude more common than mages after the above issues are weeded out. Pretty sure that's just wrong math there.
The way I see people like Alex is that they're the experts, the professionals. So say they played sports instead of casting magic. Alex, Lyle and so on all play in the pro league. There are Micheal Jordans out there, but everyone is good enough to make it their life. People leading the "quiet life" are people who've played sports, but just aren't dedicated to it like pros.
It seems like the divisions between the three families of magic is a lot more sharply defined that the divisions inside those families. So the ratios of specific magi is probably something that can remain fairly vague (given that no two diviners will be capable of the same things, despite sharing the same general classification).
I could keep going on with random thoughts, but I suspect more directions would probably be better. I've typed 5 different questions but none of them have seemed right! Hmm...things to figure out:
- Ratio of normals-sensitives-adepts-mages
- Mortality rate of developing mages
- Percent of mages that take an active part in the magical world (aka pro sportsmen)
- Ratio of the three magic families
Let's work with the population of London at ~7.8 million people. For the sake of establishing ratios, for now lets ignore the fact that large metropolitan areas, and London in particular, likely have a higher concentration of magic-users due to history and government.
Ratio of normals-sensitives-adepts-mages
- The normals is a pretty obvious number, no work needed there.
- 10% of the population being sensitive to magic seems just fine - plenty of people believe in magic as it stands, so this wouldn't affect the "normal" part of Verus'verse all that much.
- How many people in London would likely be adepts? Using my ratios above, there would be 1 in 10,000, or ~780 adepts in London. This would mean that you'd have a decent chance of having met at least 1 adept, but it wouldn't be uncommon for you to not know any. Moving it to 1 in 1,000 means there would be a solid-to-good chance that you know at least one person that is an adept. Using your initial figures of 100 to 1 pretty much guarantees that you (as a random normal) know multiple adepts, as London would be populated with 78 thousand adepts.
- How many people in London would likely be mages? Starting with your initial ratios and figuring in the estimated 10% loss, there would be roughly 7,000 mages living in London (including children and elderly). So roughly 1 in 1100 people is a mage, meaning John Smith is likely to have met at least one. Stepping up in rarity to the middle ground of 700 mages in all of London means that you may have met a mage, but the chances of knowing one is on the lower side of things. Personally I could see 700 mages living in London but only if the location is factored in (though it still feels high). Reducing to my initial ratio theory, we'd get ~70 mages in all of London, making the chances of knowing one pretty low.
Mortality rate of developing mages
Your initial suggestion is 10% of mages don't make it into adulthood. This seems like a solid jumping off point. It's certainly dangerous, but not enough to majorly affect things. You could exagerate this significantly if you wanted unguided maglings to have a lower chance of survival. It could even be something like a +50% mortality rate for solo maglings, particularly if the normal-to-mage ratio is high enough.
Percent of mages that take an active part in the magical world (aka pro sportsmen)
Presumably a low number, 5-20%. Most people are just going to want to live their lives. And the cream of the crop needs to rest of the crop to be a large majority to really stand out anyways.
Ratio of the three magic families
Totally up to you, and by far the easiest for you to just decide.