I wonder how that percentage increases or decreases over the course of his career.
I felt that when he was dominating early in his career he was hitting a lot of winners off his backhand drive. Still using the slice to draw guys in, but then magnificent passes from either his FH or BH.
As he aged, I felt he leaned a little more on the slice to help find ways to get to a Forehand.
My guess is in his early years less slicing compared to his latter years.