...which becomes compounded by the crew doing its own timing on the stopwatch.
Quite frankly the tower controller has quite enough to do rather than clocking 120 seconds, whatever, on behalf of the crew, so since the crew are the first at the scene of the turbulence they might just as well do the fine tuning themselves. They know the required minima too...don't they...
The crew would certainly have to do it at an aerodrome with no ATC!