Well, I don't know the exact values that WIS uses .. but I use 5 points for each so for me:
B+ : 85
B : 80
B- : 75
I don't think it really matters that much. In the places I use IQ, it usually means 10% of the overall score. So the difference between 8.00 and 8.45 (the average if you use 83 to 86 instead of 80) is not that much. Also, so long as all the players are graded using the same equations, then relative to one another the impact is still the same.