Canning’s drawing had nothing to do with the May 1964 re-enactment using the QM.
The only connection I presented was that they both had Kennedy as far right as he could go. However, it's hard to believe that Canning never reviewed the only incremental-stop-go-frame-by-frame reenactment.
Canning’s position was based on his analysis of Betzner’s photo in which Canning concluded that the view of JBC was blocked by the man standing in front of Betzner. He concluded, incorrectly as he later admitted, that JBC’s right shoulder was left of the right side of the ‘blocking man’. He simply extended that sight line from Betzner past the right side of blocking man to the car and showed it on an overhead view of the actual presidential limo. He placed JBC’s right shoulder to the car-left of that line.
Canning refers to a single sight-line, but it's rather wide. In Betzner, Canning's wide sight-line goes from the handhold to the base of the rollbar. On close inspection, the left edge of the sight-line meets the base of the rollbar; the right edge goes by Kennedy's left shoulder.
Connally could not be seen in Betzner, but his absence means he is inboard relative to Kennedy. There's enough space between blocking man and JFK that, if Connally was not to Kennedy's left, a portion of Connally's head would have been seen. Canning made reference to "a similar angle in a photograph taken by James Altgens on Houston less than a minute earlier."
He evidently thought the Altgens Houston photo showed a small gap between Kennedy's left shoulder and Connally's right shoulder when looking from the trunk. I have tried to approximate what I think Canning might have visualized (kind of like how you visualize your trajectories as if they're real). On the right inset, even without correcting the plat's proportions and by just adjusting the parade bar, Connally begins to move rightward into his seat.
Thus, Connally is significantly inboard of Kennedy while on Houston. Since none of Connally's head appears in Betzner, then he must still be as inboard relative to Kennedy as he is seen to be in the Altgens Houston photo.
Canning also noted: "Connally cannot have been sitting very far to the left of this position in view of his location in Zapruder frame 190." So Canning wasn't just blindly placing lines on Betzner only.