The review system on the site is every movie gets an A, then I get cold feet and add a minus on the end.
Seriously though, I think the scale is pretty much as you have it. Ultimately it's instinctive. I always preferred grades myself, ever since the days of reading Entertainment Weekly in a US supermarket in '99.
Note: F is a wildcard reserved only for Dalmatian Jaws:http://www.movie-moron.com/?p=455
I can change the Iron Man 3 rating if you want, the only people who will notice are folk who read it twice and there aren't many of those. Your review is much more in keeping with critics than my rant, I'm just mouthing off about what I see as the movie's faults, not at your review which is excellent.