There is a definitive answer, but it's a bit hard to understand, apparently
If your measure from the end of your robot arm to either end of the back bumper, and it is more than 80", then you violate the rule.
In the case if the end of the arm is just under 80" from the center of the rear bumper, and the arm extends straight forward from the center of the robot, it would voilate the rule when measured from the ends of the bumper.
Make a sketch....post it...we're very good at arguing about stuff we can see.