The first shot was fired around Z157. Look at the photos below within a second both Connally and Jack Ready quickly turn their heads to the right.


Starting at Z-150, Connally, who had been looking at the spectators on the grass to his right, quickly and consciously (i.e., not as a startle reaction) turned his head to his *left* before that. That's why he appears to be looking almost straight ahead in the famous Croft photo which was taken at Z-160 (iirc).
Jack Ready had also turned his head to his *left* about the same time.