Your calculation is the total distinct subjects (11) divided by the distinct students (15), giving 0.73.
Changing countd[subjects] to count [subjects] gives you what you need I think
No need for the table calculation.
EDIT: Retract all of that - I hadn't seen the levels, hence the countd
Okay, you still don't need the table calculation. First, let's calculate the total subjects taken by all students (regardless of the number of levels)
and then we can simply divide this by the number of students
Appreciate your help here. Is is spot on. I'd got caught up in Table Calcs and had missed the idea of counting the number of the combination of the two fields.