How do I calculate a percentage between fields in a pivot table?

I've attached a small sample sheet which shows students and their results based on different methods of teaching.
I'm trying to find an way of automatically calculating the following:
- the % of merits achieved per teaching style (excluding those that have dropped out)
- the % of passes & merits achieved per teaching style (excluding those that have dropped out)

When I try to do a calculated item it gives an error saying that I may be using the same field more than once but I don't know how else to arrange the table.
I may add additional grades into the outcome at some point so would be great if this could be captured automatically, but if not I'll just have to deal with that later.

Thanks

That works great thanks

Is there a way of combining both the merit and pass as a percentage of the total?