This is how we mixed the white coat back in my day as a plasterer. However we used a much larger mortar board, as a rule 4 to 5 feet square. We also mixed much larger batches. We would use quick set gauging plaster, but also used a retarder in order to slow the set long enough to allow or using the large amount mixed. We would mix enough to do the entire ceiling of the room, and about half way down the walls. A scaffold would be built for the entire area. There would as a rule be enough mixed to do the bottom sections of the walls from the previous mix after the scaffolding had been removed. We used both the hawks and the trowels for the mixing.

Or use a mixing drill 😊😊