It is from the judges eyes (Ok, shoulders)- we learned about this in the judging class at the beginning of the season. The AMA book says for reference, the judge should extend his arm straight out (parallel with ground) then straight up (vertical), then split that into 3 parts (30,60,and 90 degrees). It is definitely not an exact reference, and I don't think it's really meant to be. You shouldn't be straining your neck looking up at the airplane, and you don't want to squeeze your sequence, vertically, too close to the ground.