I agree and think that GRIP needs to be defined as: "Any finger/thumb that is penetrating the original surface of the ball (i.e. by a finger hole or thumb hole)".
Centre of GRIP: "The centre-most point of all finger/thumb hole/s (combined) being used at the time of delivery" *insert diagram here*
Centre of GRIP: "The centre-most point of all finger/thumb hole/s (combined) being used at the time of delivery" *insert diagram here*