JBC told us why he turned his head. I stated Rosemury Willis turned back to the TSBD because we see her do that and years later said she saw pigeons fly off the roof. It is a logical inference that the reason she turned back to the TSBD is because that's where the sound seemed to come from.
JBC said he turned to look at JFK. He doesn’t even try to look at JFK before z230. So your conclusion that this turn occurred before z230 is not based on evidence.
Rosemary turns back at z202-206. That suggests the first shot just before then, just as her sister, father, Croft and Betzner, the 25 witnesses who reported seeing JFK react to the first shot said. That’s the evidence that the correct interpretation of the zfilm should fit.