Bonnie Ray Williams had to have left the SE 6th story window by 12:22:30 so as to have reached the elevator and be off the 6th floor by 12:23, so as to allow Oswald, the presumed shooter at least 30 sec to walk onto the 6th floor from probably hiding on the 6th floor staircase midway up, and then 30 sec to select which boxes to stack and place one box on the window ledge by 12:24 so the box is seen in the Bronson film at 12:24
But if Jarman is correct that he and Norman were at their windows when they were joined by BRW only a "few minutes later", that would mean Jarman/Norman had to have left from out front of TSBD on street level by 12:18 so as to be at their windows by 12:20. thus "several minutes" earlier than BRW joining them at 12:23
However, what reason would Jarman and Norman have for leaving as early as 12:18 if the radio transmission imforming of JFK limo approaching was not until 12:22pm?