I'm going to have to think about this, but my initial reaction is that you're right and that the ratio I was using is too high.
Since you've done such a thorough job on it, I hope you don't mind if I enlist your help in coming up with a better figure. Here's some extra information which I haven't written down up to this point.
The first mistake (which I'm going to have to correct) is that the numbers I was using shouldn't be referring to the active population: they should be referring to ability at birth (or, if not birth, the point at which magical ability develops, which is the subject of another article later). So out of the fraction of potential mages, the majority get whittled down by:
• Death: This is something that I haven't openly stated but which is kind of implied in the books - mages and adepts have a much higher rate of violent death and/or insanity than normal people. In particular, many die young due to being preyed upon by older mages or apprentices.
• Quitting: Largely as a result of this, a lot of potential mages and apprentices make a conscious decision to abandon their talents and shut themselves off from the magical world. Less chance of madness and a higher life expectancy.
• Quiet Lives: Mages who don't want to take the previous option but don't want to be at risk of the first option usually choose to lead low-profile lifestyles. They don't get involved in magical politics or conflicts, avoid drawing attention to themselves, and generally make it hard for people to identify that they're mages at all.
I haven't worked out exactly what fraction of potential mages fall at one of those hurdles, but my first guess is that it's around 10:1. So the ratio of potential mages to normals would have to be higher than 1:100,000 to support the death/dropout/quitting rate.
The ratio of diviners to other mages . . . hmm. I haven't decided on it for sure, but your guess of 0.5% isn't a bad one. I'd say somewhere between 0.1% and 1%.