Here is a compilation of frames from Dorman's film (credit: John Dolva) that I found at the photo gallery here:
The bottom compilation is helpful in visualizing the view that Victoria Adams had. Sadly, Dorman didn't film the actual motorcade, but this lets us verify the relationships of positions of items in our 3-D computer models when viewed from Adams' position.
Also, as we watch the actual Dorman film pan from left to right, tracking just above the limo, when the film reaches a point in time right between the end of Towner's film segment (D335) and the beginning of Zapruder's film segment (Z133) there is a sudden apparent jerk of the camera for one frame before a stop (equivalent to the time it would take for 49 frames - or 2.97 seconds) before beginning again. After the Dorman film restarts, it is no longer tracking the President's limo. Instead, it is panning back and forth haphazardly apparently frantically searching for what had just caused the loud noise and the following two loud noises.
Here is a post that I made a while back showing the occupants of the limo during the first part of the Zapruder segment along with a description of their actions:
In the clip below, I believe that you can see each of the limo occupants simultaneously react to the sound of the first shot.
JFK suddenly drops his right hand from his hair and looks to his left (towards Jackie). Then suddenly returns his head to the crowd on his right (probably reacting to the sound of someone yelling).
John Connally first turns to his right, then quickly turns to his left (towards Nellie), then back to his right.
Jackie suddenly turns her head further to her left, then back towards the front of the limo.
Nellie Connally suddenly turns to her right (towards John Connally).
Agent Roy Kellerman suddenly turns his head to his right, then quickly back towards the front.
Agent Greer (driver) appears to have his gaze towards the camera (Zapruder) and doesn't appear to move his head. He is the exception, perhaps Zapruder (up high on the pedistal) caught his eye.
And last, but not least, William T. McIntyre (on the left running board of the presidential follow-up car) Hickey suddenly looks to his left and down. He appears to me to be looking at the tires of the limo for a blow-out. (Perhaps he thought the loud noise might have been a tire blowing out.)
Edit: William T. McIntyre is almost hidden behind Clint Hill. However, I believe I see him suddenly duck his head and look around Clint Hill.
While there isn't a way for me to accurately model (100%) that tree as it existed on 11/22/63, I can model one that should be similar to it. And this experiment does show that the limo should have still been behind that tree (from Adams' viewpoint) at the Z160 point in time. So, her account doesn't suggest that the Z160 point in time was out of the question. This experiment suggests that JFK should have reappeared from behind the tree (from Adams' viewpoint) by approximately the Z180 point in time. Therefore, based on Adams' account, I believe the first shot must have occurred before Z180.
However, when combined with the other accounts (Brennan, Towner, etc.), and visual items (Dorman film stop, limo occupants' actions, etc.) it appears to me that the most likely timing of the first shot is in the 0.82 second interval between the end of Towner's film segment and the beginning of Zapruder's film segment. See Dale Myers' synchronization of the films for references.
