If you want an ABC ranking, you'll be going by 5's.
See, schools have it unfairly ranked against you--in a test with ten questions, missing three is a 70% (that's basic math). That's a C, which is considered average. So, an average student has a margin of error up to 3.
Even if you get half the test right, you still fail it. In a logical world, getting half the test right would equal a C, which is the "middle" grade. However, because schools demand their students do well, they screw the system up in order to force students to work harder. Those who do not are eventually weaned out and are deemed to be failures of the system.
This is a double-edged sword, because it works well for some kids and doesn't for others.
However, if you try to apply that to video games, you're going to realize that it doesn't make a whole lot of sense. If you have 5 letter grades--F, D, C, B, A, then you're going to be able to evenly match them up to one another through a 5-point number system (five stars works too.)
1UP has gotten it pretty good--it translates it's ten point system into A, A-, B, B-, C, C-, etc.
I'm not sure if they incorporate A+'s or B+'s, but if they did, that would make it something of a 15 number system, which screws my math up.