"AA" grade shooters are those who's average score is above 80%
"A" grade shooters are those with an average score between 70% and 80%
"B" grade shooters are those with an average score between 55% and 70%
"C" grade is for shooters with an average score up to 55%
A grade is 5% narrower than the other bands seems to be a reasonable argument to extend it to 85%.
Depth is AA is also going to be a reflection of attendance at regional shoots, where lets be honest a 30 shot winter league is quite possibly going to leave you a higher %age than a 50 shot GP course where there are 20 more targets to potentially miss.