Obviously you cannot have a straight grading system where 90% is an A and 80% is a B and 70% is a C, etc., because then 423 out of 500 ranked NBA players are failing (below 6 out of 10, even rounding up).
So you need to grade on a curve. There is a more statistically sound way to do this, but it would take more effort, so I am going to go with this idea:
Reduce the league down to 360 guys (30 teams times 12 active roster spots).
20% Ds and
So here are your cutoffs:
Right at the D to F cutoff (around 3.02 out of 10):
Right at the C to D cutoff (around 3.8 out of 10):
Right at the B to C cutoff (around 5.45 out of 10):Baron Davis
Right at the A to B cutoff (around 7.26 out of 10):
A+ (above 9.65) -- Dirk, Paul, Wade, Howard, LeBron