This is about the best interputation of the system that I have found and agree with, but everyone will have thier own Ideas to dispute this
This is an excerpt of an article by Bob Cambell in All About Pool magazine (December, 1997) giving guidance on handicapping nine ball players in a race to seven:
D- Player
>will not run a rack
>average run is about 3 balls
>with ball in hand, will get out from the 7, one out of 3 times
>rarely plays a successful safe
C-Player
>will probably run one rack, but usually not more than one
>avg. run is 3 to 5 balls
>with ball in hand, will get out from the 7, two out of 3 times
>mixed results when playing safe
>inning ends due to botched position, missed shot or attempting a safe.
B-Player
>Able to run 1 to 3 racks
>avg. run is 5-7 balls
>with ball in hand will get out form the 5, 2 out of 3 times
>most of the time a "B" player will play a "safety" which maybe hit easily 2 out of 3 times
>a typical inning will end with a missed shot, a fair safety, or a won game
A-Player
>will string 2 to 3 racks
>avg. ball run, 7-9
>with ball in hand, will be out from the 3 ball, 2 out of 3 times
>typical inning will end with a well executed safety or a win.
OPEN-Players
>average 8+ balls
>string racks together more than once in a match
>is a threat to run out from every ball, from every position, every inning
>typical inning will end in excellent safety or win
Most pros have run at least 150 balls in straight pool, many significantly higher, and many will run 80 balls or so fairly often.