In my mind, when I think of A-D, I think of what level of ghost a player can beat in rotation (not once, but more than half the time in the long run).
Roughly, I think it comes out to about
Pro: 12 balls or more
A+: 10
A: 9
A-: 8
B+: 7
B: 6
B-: 5
C: 4
D: 3
Of course, it depends on the...