The diagrams in 'Swing Like a Pro' show apx 9 deg open at address and as noted earlier, about the same at impact. The reference line is from one shoulder to the other (vs an inference from the rotation of the thoracic spine) as was the case in the study I cited earlier with used the tips of the acromions
. For more specifics you need to have the model swing software.
Since you seem to be interested in dueling measurements what does Dr. Neal say is length of the upper arm, lower arm, thigh and leg and torso width from his 'measurement' of tour pros? With Dr. Mann all of those measurements (and more) went into the creation of the model pro (10.8", 10.3", 14.8", 17.8" & 19.9").
All of these measurements would be taken of you in your lesson and the model adjusted to match your body. Then your actual performance of the swing could be overlayed against the model pro so you would see exactly where you were different in EVERY detail not just segmental rotation.