You don't need to interpret the Z Film. The eyewitnesses state exactly where the first shot happened. The Chisms state it happened right before the car gets to them.
John Chism : "And just as he got just about in front of me, he turned and waved at the crowd on this side of the street, the right side; at this point I heard what sounded like one shot,"
It's just that a frame like Z162 is "just about in front of me".
Chism would be watching the car as it neared him. The Willis photo is about two seconds later and the Chisms appear pretty close to the car, so certainly some looking up towards the car before that.
However, Chism seems to recount just two shots:
"And just as he got just about in front of me, he turned and waved
at the crowd on this side of the street, the right side; at this point
I heard what sounded like one shot, and I saw him, "The President,"
sit back in his seat and lean his head to his left side. At this point,
I saw Mrs. Kennedy stand up and pull his head over in her lap,
and then lay down over him as if to shield him."
In that case, the "first" shot is the one before the head shot. He's describing as the "first shot" the SBT shot in the Z220s. Another thing, too, Chism wasn't sure about the number of shots, allowing for up to three. But he could only really recount what happened during two of those.
If the first of the three shots was the one he lost track of, it's possible he heard it while the car was further up Elm.
His wife, Marvin Faye, is also "first shot" before the head shot:
"As the President was coming through, I heard this first shot, and the
President fell to his left. The President's wife immediately stood over
him, and she pulled him up, and lay him down in the seat, and she
stood up over him in the car. The President was standing and waving
and smiling at the people when the shot happened.
And then there was a second shot that I heard, after the President's
wife had pulled him down in the seat."
Likewise, Jean Newman. So basically, their statements could be a way to situate the moment of the second shot, rather than the first. The general consensus is that three shots were fired, with the head shot being the last.
The eyewitness accounts are interesting but often not reliable. I look at the film for subjectivity (yes, film and digital recording can be manipulated, but this one seems accurate).
The turning of the heads of the Connallys and Mrs. Kennedy just as Rosemary Willis begins to slow to stop: that's first shot territory. The simultaneous reactions of both men in the Z220s: that's second shot territory. The head shot: obvious, but the head initially goes forward first, so consistent with SN.