Well, I still haven't heard back from anyone, but was reading an unrelated part of Toole's book again, and saw some indirect further fuel for the fire. Toole seems to understand the Early Reflections curve to only include a single curve from the rear hemisphere, the 180 degree one (+/- 90 exists in both hemispheres, of course).
"If the loudspeaker under test is a horizontally symmetrical forward-firing design the early-reflections calculation involves only 18 measurements to embrace the floor, ceiling, one side wall, rear and front wall reflection estimates. Asymmetrical designs require both side walls and both left and right front -wall data points, bringing the sum to 26 measurements. All but one of these (the 180 ° measurement) are in the front hemisphere."
(Page 132 of the third edition)
So, this once again points to the only rear data being included in the ER curve being the +/-90 and 180 degree curves, not an average ranging from 90 to 180 in 180 degree increments.
Also note that the above quote also seems to include a typo. A horizontally symmetrical speaker should only need 17 measurements, not 18: 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, and 180 horizontal, and -20, -30, -40, +40, +50, +60 vertical.
Since Toole seems to think the ER curve is calculated with just three rear curves, that's what I'm going with unless told otherwise. Still doesn't clarify if the measurement is an average of averages or a simple average of all the measurements though.
