At the moment, the limit is global, as you’ve noticed. So you’ll have to figure out how to break it up on a per-calendar basis and figure out how to manage each one, how to determine if the user wants a global, or individual, how the individual one is setup, which calendar it applies to, etc., etc. Or maybe you will come up with a whole different method … :)
I’d tackle this, however my free time has been severely limited lately …