I am not sure the FAI-F3A rules actually define how the top of the box is determined (exactly), although it would make sense that the reference point for the 60 degree 'top of box' would be at the point where all defining lines for the 'box' meet.

i.e. It would be odd that there are lines defining 60 degrees to the left and right joining the center line at the pilots feet . . then measure the 60 degrees 'top of box' angle from somewhere else (e.g. the judges who are to be seated not more than 10m, and not less than 7m behind the competitor’s position (the apex of the 60 degree lines) and within an area described by the extension of the 60 degree lines to the rear of the competitor . . now there's another variation right there).

I reasonably agree with LLD . . the difference would be of little consequence in the subjective world of judging .

Best to make sure we fly reasonably within the 'box' or risk downgrades . .

Cheers, JB