This simple and common sense point is somehow overlooked by those trying to push an early missed 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.