If the shooter is sitting in the box by the pipes which Charles has shown is plausible it really does not make much sense the shooter standing up intentionally trying to shoot an
aimed shot as early as Z130 or Z140.
If the premise is that boxes were arranged as they were to be a firing platform then the shooter was going to simply lean over once the limo has passed by and to start shooting from this platform.
So the only other option is for a missed 1st shot that precedes Z224 is the shooter inadvertently squeezing the trigger AS he leaned over from the box he was sitting on, and that the rifle angle was not aimed at the limo.
As much as I agree that scenario is plausible it’s not probable imo. A 1st shot at Z130-Z140 to give the necessary gap longer between 1st and 2nd shot fitting the pattern of 1…2..3 would lengthen the Total time for completion of all 3 shots in 9-10 secs which is WAY longer than Harold Norman heard, and the smaller gap between 2nd and 3rd shots being 4.8 secs which is WAY longer than “back to back”.
I don’t disagree with Dans idea that the 2nd shot could be 313 , and that the shooter in his crazed mind being “exuberant” that he had scored head shot fired one more shot rapidly without aiming ( thus possibly only 1.5 sec later if using the MC rifle)
It’s just that there are so few witness recalling any shot after Z313.
If the premise is that boxes were arranged as they were to be a firing platform then the shooter was going to simply lean over once the limo has passed by and to start shooting from this platform.This simple and common sense point is somehow overlooked by those trying to push a ridiculously early shot.
The Sniper's Perch is the arrangement of boxes supposedly used by the shooter to rest his rifle on during the shooting.
This means the boxes were arranged BEFORE the shooting.
This means the shooter had already planned out what he was going to do BEFORE the motorcade arrived.
The arrangement of boxes tell us the shooter had decided that the 'kill zone' was going to be towards the triple underpass.

This is confirmed by the testimony of Ronald Fischer.
Fischer and his work colleague, Bob Edwards, were stood directly across from the TSBD building on Elm Street waiting for the motorcade. Edwards noticed a man on the 6th floor who seemed to be in a cramped, uncomfortable position, he pointed the man out to Fischer and made a joke about him. Something about the man made an impression on Fischer:
"And I looked up and I watched the man for, oh, I'd say, 10 or 15 seconds. It was until the first car came around the corner of Houston and Main. And, then, when that car did come around the corner, I took my attention off of the man in the window and started watching the parade. The man held my attention for 10 or 15 seconds, because he appeared uncomfortable for one, and, secondly, he wasn't watching-uh---he didn't look like he was watching for the parade. He looked like he was looking down toward the Trinity River and the triple underpass down at the end-toward the end of Ell Street. And--uh--all the time I watched him, he never moved his head, he never-he never moved anything. Just was there transfixed."This is pretty much what the shooter was staring "transfixed" at:

The shooter knew exactly what he was going to do before the motorcade entered Dealey Plaza.
The area circled in the pic above is where the limo is moving almost directly away from his position. There would have been minimal lateral movement of the target.
He would have been aware of the oak tree and would have taken his first shot once the limo had moved into the open space as shown above. Once the limo had cleared the oak tree he would have had more than enough time to get multiple shots off while the target was in the 'kill zone'.
The idea that he was on his feet trying to shoot through an impossible gap in the half-closed window in an area where he had nowhere to stand is plain stupid.
He knew when he was going to take the first shot before the motorcade reached Dealey Plaza.
That he was going to wait until the limo was in the kill zone circled above is yet even more evidence that the first shot was taken at z222/z223.