Thanks for the further explanation, David. I will follow up your suggestions and play around with the grading until I get a satisfactory result.
I understand from previous threads that the idea of a separate teacher's grade has already been discussed, but I still think it would be worth reconsidering. Just as the "Allow self-assessment" check box, there could be one for the teacher's grade. The teacher's grade (a standalone, unweighted figure) would simply be added to the more complex peer assessment grades, giving a final total grade, for example, out 100.
Would you consider this? I think it adds a useful extra functionality to the workshop module, by allowing joint peer and teacher assessment.