Working backwards from the opening hour
The first number to establish is not the start time but the finish. If traders begin arriving at five, then floors must be dry and equipment gone by then, which puts the last wet task somewhere before four depending on the surface and the airflow. Every other task is then positioned behind that point, and the shift start falls out of the arithmetic rather than being set by habit or by what suited a previous arrangement.
This ordering exposes the tasks that do not fit. A full machine scrub of a large trading floor may need more drying time than the window allows on an ordinary night, which means it belongs on a closed day or a reduced-trade day. Discovering that during planning is straightforward. Discovering it at four in the morning with a wet floor and traders at the door is not, and it tends to happen more than once.
