I suggest to put into use one more way to calculate the task duration for tasks with "Work" Task Type.

For this we need to implement one more Duration Type - "Physical Quantity and Resource Units Dependent" and one new parameter of Human Resource and Equipment is "Productivity". "Productivity" is measured in "Physical Quantity per hour". And in that case the duration will be calculated in the following way:

"Duration" = "Physical Quantity"/ ("Productivity" * "Resource Count").

That's very natural way to calculate task's duration and that's the foundation of Resource Planning.