Measurements might not have been precise. That doesn’t bother me. If one believes the head shot came from behind then Oswald likely did it.
My problem is I have never been able to watch the Zapruder film and feel convinced the shot came from behind based on how his head reels backwards. Perhaps the car was speeding up at the exact moment the head shot was delivered. Dunno.
Best explanation i've heard is that when people get shot in the head, they sometimes "twitch" or "spasm" violently. I believe years ago I saw some documentary which stated this was the cause of his backward motion.
Besides, entrance and exit holes indicate direction of travel. Little hole for entrance, big hole for exit is the norm.