First, the simultaniety of those two shots was established by the Warren Commission who said they were one and the same shot. Jim and others rejected the claim of one shot, but kept the simultaniety, claiming that it was two shots that struck at the same time. I don't think Jim claims exact simultaniety, but rather that they occurred so closely in time that they were practically simultaneous. But, even that is wrong in my opinion. Now, let's move in closer on that spot.
So, if JFK was hit with both the back and neck shots at that point, he should have begun reacting to both shots then and continued reacting until the fatal shot. The shot to his throat obstructed his airway, but he apparently cleared it by coughing, as we see in the Zapruder frame 230.
This was a very immediate response to the shot to his throat. It was fast, like a reflex. His hands went up in less than a second. So, when did the bullet strike? Less than a second before. If it was two-thirds second, which I think is enough time to give it, it would be frame 218, though I realize that does not account for frames that were removed. But, for now, let's just say 218.
But, what about the back shot? Last night, on the JFK show, we all agreed that Kennedy was hit in the Croft photo, and that subtle and elaborate alterations were done to hide that fact. So, let's add the line for the Croft photo.