Hmmm...
If JFK isn't clear of the foliage until z207 and the graphic Jerry posted above has JFK still in the foliage at z186 this surely means a shot at z190 would have been through the foliage.
Am I missing something?
The FBI conclusion that JFK was not clear of the foliage (that is, the foliage in late May 1964) until z207 is based on their re-enactment using the Secret Service car (the "Queen Mary") from which this photo is made:
Even based on that photo from the FBI re-enactment, we can see everything back to the rear of the car which was about 7 feet behind JFK. So JFK would have been visible 7 feet earlier. At a foot a frame, that brings it down to z200 even in the re-enactment.
But it is not really an accurate re-enactment. The foliage Oswald saw was similar to that depicted in the December 1963 Secret Service film which is somewhat less full than in May 64. The president just had to move past the centre of the outer branch to be quite visible:
Also, because they used the wrong car, you can see that JFK does not appear in quite the same position from Zapruder's position in the reenactment:
So, in my view JFK was in Oswald's sights several frames before z200. I may be over-stating it a bit at z190 but I think it is safe to say JFK was clear to Oswald by z195. But more significant perhaps is the fact that the tree branches never completely obscured Oswald's view so Oswald could have sighted on the edge of the leaves, tracked the car aiming at a point about 2 feet in from the right side of the car and fired as soon as JFK was just about to come into the sights or cross-hairs (depending on which he was using).