Imo, if there’s a 1st shot fired as early as Z150-160, then the reason it was a complete miss was because the shooter , sitting on the box , (as Mr.Collins demonstrated in his self built full scale model of the SN), maybe had to adjust himself more than he anticipated and he in haste fired the 1st shot before he had his rifle securely resting on the top of the box on the window ledge.
Perhaps he leaned over at about Z140, rested the rifle on the window ledge box, and then saw when he looked thru his scope or his iron sights that he couldn’t get JFK in his sight without partially standing, so being in a state of anxiety already. he tried to stand up partially and take a shot and in doing so, maybe he did not lead the moving target enough, since the limo at Z150-Z160 being a closer angle requires a faster tracking speed than at the longer angle at. Z224.
Unfortunately the CBS trial shooters did NOT replicate this theoretical firing position that Mr.Collins experiment shows is reasonably plausible.