A Ball's Width
As can be seen in this diagram the red ball not in shadow is fully snookered behind the blue ball with approximately the same distance from the cue ball to the blue as from the blue to the red. Both the cue ball and the ball on are in open areas of the table with the cue ball in a position that would not cause awkward cueing in any escape attempt.
With the width of a ball being 52.5 mm I will refer to this as one unit. The rule book states that to avoid a call of ‘foul’, a ball or balls on must first be struck by the cue ball. It does not say that any such ball must be struck centrally and if contact is made at the extreme edge of the ball on by the extreme opposite edge of the cue ball then no foul has occurred. This, gives a target of the full width of the ball on and with the full width of the cue ball to play a legitimate stroke. The target area for an extreme edge of the cue ball is thus stretched to about 104 mm, so not just a 1 or 2 mm target point.
In the case of the cue ball failing to contact the red in this example the following should be considered. The red balls in shadow surrounding the full colour red represent a distance of one-unit error margin in any attempt to strike the red. That representation shows a significantly large target area on the diagram and is just as true practically. Considering this area, I would be hard pressed to think of any sort of competition player, from professional to novice D grader, who would struggle to strike it even at a first attempt. This therefore, may be taken as a fairly accurate guide when assessing whether or not to call a miss if an attempt fails. I would even go so far as to suggest that the most that novice D grader could fail by to avoid a call of ‘miss’ would be no more than one quarter of a balls width either side and that higher skilled players must be expected to complete an escape in this case.
Lots of other different factors of course come into play when the escape route and/or cueing are not so straight forward, the cue ball under a cushion, the cue ball hampered by balls not on, the cue ball in close proximity to balls not on and causing the snooker such that judgement of the escape route is difficult, the ball on in close proximity to balls not on, less than a full ball available to be struck, the cue ball caught in the jaws of a pocket, different balls not on covering different escape routes, the proximity of pockets in a direct line on the natural escape route with the possibility of side spin being required and the necessity of using more than one cushion in an escape attempt, would all be examples of more difficult snookers.
Regardless of the ease or difficulty of any escape, by using the unit to assess how close the cue ball came to a ball on when missing, can often be the best guide in making a decision. I would suggest a maximum of three units for the most difficult of snookers being attempted by lesser skilled players, with fewer, or fractions of fewer units for those of less difficulty and/or made by higher skilled players and on a sliding scale down to the one described in the diagram. This would be reasonable in 99% of cases, the impossible snooker notwithstanding.
Below is a representation of 1, 2 and 3 unit distances from a ball on in all directions.
It must also be considered that easier escapes are sometimes available rather than the one chosen. It may be the case that in playing an easier escape the striker will more than likely be leaving an easy pot for an opponent and in choosing the more difficult option is at least confident of leaving the cue ball and any balls on in safe positions, even if first contact is not achieved. A typical example would be trying to just contact a single red on or near a cushion instead of targeting a group of red balls that might still be a collective at or near the start of a frame. Missing in such a situation regardless of how close the cue ball came to striking the red would still be a justification for the referee to call a miss. Referees must be vigilant in all such cases in not letting players circumvent the rules to gain an unfair advantage.