In this question first of all I need to point out the insignificance of this rule. It does not influence any of the boat’s performances, it is only an aesthetic option.
If I were to decide, I would delete A/13 completely from the specifications and let everybody use their imagination and skills to fulfill their creative ambitions and as it fits to serve their artistic thinking.
It is the only part of the DN that can give any personality to it and distinguish the different design styles.
In my interpretation, the current rule A/13 is a minimum area that can be placed anywhere on the backseat’s centerline. If the backseat is larger than the minimum, the above the deck area can be considered the backseat support that has no dimensions specified.
My interpretation of the current rule is:
If the backseat’s flat area at any point on the centerline is larger than 11″ in length 4″ in width and covers radius 2″ on the top side until the minimum length, it fulfills the rule A/13.
The intention of the rule is not very clear, some say it was a safety issue, some say it has something to do with hiking at the back when very hard masts were common.
I hope in reality we are not overthinking this issue.
In Jeff’s list, point 4 has a problem by overruling the specifications, where there is no mention of the rear deck level. It would require a ballot to change the specifications.