Ghost Ball and the Break

You find out where a shot goes by watching it. A player sees the ghost ball first: the spot where the cue ball will stand when it touches the object ball.

Run the step ahead

Give the cue ball the speed and spin the cue would give it, stand every other ball still, and run the table's own step forward until the cue ball first touches something. Where it stands then is the ghost ball. The object ball leaves along the line of centres, and a stun cue ball along the tangent line, square to it. A rolling cue ball starts out on the tangent too, then bends forward as its topspin grips the cloth. At a half-ball hit:

                             e = 1     e = 0.95
object ball, off the aim     30.00°    30.00°
tangent line                 60.00°    57.52°
rolling cue ball ends up     33.67°    32.26°

Players call it the 30° rule; 30 is the round number.