Thompson got it right in this book. For some odd reason, he went back on it years later. But he had it right. The six-second sequence of the shooting is spot on. You can see it immediately after Kennedy is seen reappearing from the sign. Then it goes on to Z313 which is about 6.2 seconds or so.
There's been a lot of craziness in the coming years about shots way up earlier to justify the Tague chips shot, so-called sparks and so on. It's all baloney. Ask yourself - if there was a conspiracy (and I do believe there was) would the shooters have started to fire when they knew there was going to be an oak tree there? No, they wouldn't. Think of the zone like a box - as soon as the car was in the box - that's when the shooting starts. Exactly where Thompson said it was. It's not hard to figure out.
It sometimes helps to do the math.
6.2 seconds before z313 is: 313 – 6.2 * 18.3 which is about z200 (199.54).
Even 6.0 seconds before z313 is: 313 – 6.0 * 18.3 which is about z204 (203.20).
According to Don Roberdeau’s map, the tree is in the way of a shot from z166 through z207. So, if your scenario is correct the first shot was fired while the tree still blocked the line of sight from the TSBS sniper’s nest.