Snooker rule: "When the nominated colour is potted, the player will be awarded the correct number of points. The colour is then taken out of the pocket by the referee and placed on its original spot. If that spot is covered by another ball, the ball is placed on the highest available spot. If there is no available spot, it is placed as close to its own spot as possible in a direct line between that spot and the top cushion, without touching another ball." So, how close is "as close as possible without touching another ball" in terms of physics?