Of course that setup isn't good for all speeds and CB rotation - it works well for medium speed rolling CBs and decent equipment. Mainly it's simple to measure.Pat -- the point on the head string to place the base of the CB for a 30° spot shot to center pocket is 10.44" off the side rail ignoring throw. So the placement you show in the diagram, 10 1/16", doesn't allow for much throw (less than ½°).
I have much more success on spot shots by placing the CB a couple inches closer to the side rail. Allowing for 1°, 2°, 3° or 4° of throw would put the base of the CB at, respectively, about 9.5", 8.6", 7.7", or 6.7" off the side rail.
pj
chgo