I think the tested headphone positions should reflect the perhaps small differences in mounting that we might see when we put our own headphones on our heads, because you can kind of feel how they feel on your head (you can even take a quick look in the mirror to see if the earcups are too high or too low, but most of the time doing it by feel provides the same position). I think an average of those small differences in positions has got to be the most reflective. We can't do anything about the variation of our own ears to that of the mannequin head.
It looks to me that we need to spend quite a bit more effort in the selection and calibration of microphones used in flat plane headphone measurement. With some random in-ear microphone we will get some random calibration curve. With random calibration of the flat plane measurements we will have random comparisons with the calibrated 45CA curves.
We will not know if the measured curve changes are caused by the change of microphone, coupler and removal of the external auricle.
Perhaps little steps towards the goal. May be we keep the microphone and coupler of the fake ear and remove only the auricle.
I see fun steps in learning.
First I will dig into the 45CA technology and see what microphone capsule is used in the fake ear.
The 40AG microphone is used in the 45AC-9. The 40AG is a "pressure" microphone.