Right, I think I should clarify that pooled mining is different. In pooled mining you're always targeting the network difficulty but you're allowed to submit shorter chains to the pool. The ratios between lengths 7, 8, 9 and 10 should always be approximately 10 (at least for CPU miners). That's because each number has nearly the same probability of being prime after it's being sieved. Since the ratio is 10, the probability is about 0.1.
However, you can find a lot more 7-chains if you change the sieve parameters. This is what I did in my previous post. That's why the ratios are more than 10.
Anyway I have calculated money supply cap using ratio=30
here. According to it 5 years after release XPM diff won't reach 14, and the total supply is going to be ~55 Millions XPM.
I haven't checked the numbers, but the eventual total supply of 55 million sounds reasonable. Primecoin is definitely intended to sustain a healthy mining market, since only about 11% of that has been mined in the first year.