<p>Serendipity's got the idea that most +/- systems use for GPA.
The 1.0 (D), 2.0 (C), 3.0 (B), 4.0 (A) system is still the basis, but a + adds .3 and a - subtracts .3 (This leave the largest interval--.4--between the two letter grades, i.e., A- (3.7) and B+ (3.3) ).
In addition, it seems to commonly be that -s are defined as the lowest 3 percent of a grade (e.g., B- is often 80-82), the regular grade is the next 4 points (e.g., 83-86 would be a B) and the + is the highest 3 points (e.g., a B+ would be 87-89).</p>