authenticated and premium; precedence between roles?

I have users with just an "authenticated" role, and users with a "premium" role.
I want that simply "authenticated" users can create a maximum of 30 nodes of a given content type. But I want that "premium" users (a subset of authenticated, of course) can create more, e.g. 60 nodes of the same content type.

But simply creating two rules (30 limit for autenticated users and 60 for premium users) doesn't work. The lower limit (30) takes precedence.

I understand of course it's just a matter of logic; and that i could simply invert the logic (authenticated have limits of 60, and create a "not-premium" role with a limit of 30); but this will complicate a bit the logic of the rest of the application. So I wonder if I am missing something, and maybe there's an easy way "out-of-the-box" to do this with this nice module...

Any help will be, of course, appreciated :-)