The mixed workloads that have both Time Dependant and Time Independent tasks should be handled as Time-Dependant workload first. A planner needs to establish that demand and then add the time-independent hours to the curve. If we take the Butcher workload as an example, The volume of customers may require two butchers at some points in time but may even dictate that no butcher is required. Since you know you need a butcher at all times during the opening hours, you can measure the ‘free’ hours you have from the periods when no butcher would be required and compare that to the sum of hours required for the time-independent tasks. If you have enough room, then you don’t need to add hours. If you don’t have enough of these free hours, you have two choices as a planner:
The first choice is to dictate when these tasks could be done by adding hours to the valleys in the daily curve. Therefore, you keep your workload simple, easy to measure, and you make sure you have enough hours in the day to cover all the required work. The down side of this approach is that the representation of the workload is now twisted since the planner will measure the schedule against what seems to be a time-dependant workload only. So if an employee has to go home early or comes in late, that planner may take decisions based on a workload that does not represent reality.
The other choice is to have two separate workloads, one for the time dependant butcher and one for the time independent. This would represent more accurately the real workload when the schedule is measured. In counterpart, this complicates the scheduling process since the planner will now have to keep track of two distinct workloads and also be very precise on what is assigned to which employee and when. It will no longer be a simple one to one relationship between what the employee is scheduled on and what the workload requires.
Both approaches have their ups and downs. In my experience, your selection of one method or the other will depend on the size of the business (as explained earlier, the bigger the business, the more detailed it will be), the capacity of the planners, the tools available to the planners, and the capacity for the employees to understand the schedule and its details.