Ok - took a while but here is a first effort at a breakdown over the past 8 GPs
Updated with charts ...
It is broken down by numbers attending per Grade, maximum score of the day (has quite an impact on 90% and 80% figures). With numbers and percentages achieving better than 80%, 70%, 60%, 50% less than 50% and interestingly those better than 90%.
I think I found all the errors but using google spreadsheets is a learning curve ... some things paste, some don't. There may be other errors but without the original sheets it is a bit of pain to extract the data.
Not sure how it helps but at least there is some data.
All the best