Rob made a few suggestions so I have put together a slightly different spreadsheet
A Original much the same
B Added 90% column
C Re-worked to use Grades boundaries of >80%, between 70% and 80%, between 55% and 70% and less than 55% i.e. AA, A, B, C
D Then same grades distribution but using 50 as the maximum i.e. no allowance for top score of the day being low(!) just to indicate the influence of a hard day and a less hard one.
I added some highlighting to indicate things but will leave it to everyone else to pass comments along.