I went to McGill, where the grade scale is the following:
B-= 2.7 = 65-69
B= 3.0 = 70-74
B+ = 3.3 = 75-79
A- = 3.7 = 80-84
A = 4.0 = 85-100
I think if you were converted to a percentage, you would be based on your schools grade scale. They usually take the lowest number in conversion, so your B's would be translated to a 70%, your B+'s a 75%, etc if there were no number grades. McGill is strange in that some classes get both letter grade and number grade, and others only get a letter grade. My LSAC gpa was identical to my schools calculated GPA though, despite the discrepancy in the American and Canadian GPA systems/equivalencies.
hope that helps!