At my school:
A+ = 4.3 (but pretty much every OD school just counts it as 4.0)
A = 4.0
A- = 3.7
B+ = 3.3
B = 3.0
B- = 2.7
C+ = 2.3
C = 2.0
C- = 1.7
D+ = 1.3
D = 1.0
D- = 0.7
F = 0.0
As far as the percentages go, it actually varies from class to class. Some teachers give out pluses and minuses like candy on Halloween and some hardly give any. For example, in my statistics class the teacher gave out minuses only if you were at the lowest percent for a certain letter and pluses only if you were at the highest percent for a certain letter. He also rounded up any decimal of a percent. For example, if you got an 80.0%, that's a B-, however if you got an 80.1%, he rounded up to 81% and you got a B. Conversely, if you got an 88.0%, you get a B, but if you get an 88.1%, he rounded up to 89% and you get a B+. Everyting in between was a B.
This was just an example of one class, teachers are pretty much allowed to hand out whatever grade they want on any scale they want. For example, if the class average is something like a 65%, then it is likely that a score in the mid to high 80's will give you an A- or A, unless the teacher is a hard ass about grades.