You are right. At z272 the angle of Zapruder to the perpendicular to the car path is about 18-20 degrees so your 72 degrees is about right. I had in mind a 30 degree angle but that was the angle at z190. So JBC is a bit past 72 degrees but the plane of his chest (ie. the perpendicular from the chest surface) is not as much as 90 degrees to the direction of the car.
As far as the path of the bullet striking JBC:
1. it struck the fifth rib in the back close to the axilla (armpit) and imparted a significant impact that JBC described as a hard punch from about 12 inches away.
2. The impact indicates that the bullet lost some momentum on hitting the fifth rib. So it either slowed suddenly or changed direction suddenly or a combination of both.
3. The bullet destroyed the last 10 cm or 4 inches of the lateral and anterior (front) portion of the right fifth rib and emerged just below and slightly medial to the right nipple. The point of impact on the fifth rib was more than 10 cm from the end of the rib.
One can conclude from that that the bullet did not destroy the fifth rib at the point of initial contact.
4. In destroying the last 10 cm of rib the bullet drove shards of rib bone down into the lower lobe of the right lung. That means that the bullet drove bone fragments inward.
5. The bullet did not penetrate the lung.
From the medical evidence, the bullet followed along the fifth rib for a few inches before smashing through it which is consistent with some deflection of either the rib (to the left) and the bullet (to the right) or a combination of both upon impact. It exited just below the right nipple. Twist your torso around like that and you can easily see a path along the rib and exiting just below the right nipple that does not pass through the lung. In fact it is much harder to make that path work on a shot from the SN at z225 with JBC facing forward without passing through the chest/lung. When he is twisted right, the right nipple is essentially on the side and the point below and slightly medial to it almost aligns with the back entry point on a line with a shot directly from the rear without going through much if any of the pleural cavity. So there would only have to be very slight deflection of a few degrees. Try it (minus the bullet from the rear).
The angle that JBC is turned to Zapruder is not the issue.
The issue is how JBC is turned in relation to the shooter in the sniper's nest.
The pic below is taken from Speer's website and a discussion of the SBT.
It is a fair representation of the relative positions of JFK, JBC and the trajectory of a shot from the TSBD.
Obviously, as the limo travels on to z271 these positions will change slightly but the pic is useful to demonstrate two things:
Firstly, around z271 JBC is turned far more to his right than in the pic above, so much so that the plane of JBC's body, shoulder to shoulder, would be more or less in line with the trajectory of the bullet making a shot entering JBC's back, at the armpit, very unlikely:
But that's not the important point. If you look at the trajectory of the bullet it
is moving away from JBC at the time of impact.
The shot fired from the TSBD is moving laterally from right to left as it travels towards the limo. If, by some very unlikely circumstance, the bullet did strike the armpit area of JBC it would be moving right to left - away from JBC's body.
Rather than blowing out his spinal column we are being asked to believe the bullet, moving away from his body, would somehow have all the force transferred into a left to right motion through JBC's body.
Also, just from eye-balling the above diagram, the bullet would have to make an almost 90 degree turn to pass through JBC's torso, exiting the right side of his chest.
We are in dreamland now.
Finally, this mage from z280 - a full half second after supposedly being shot through the chest, smashing a rib and blowing splinters into his lung - JBC is still in the same position, looking towards JFK.
Do we really have to continue with this?