Macro to change the value of a cell based on another cell value

Hi all, I'm new the forum and looking for some help please.

Up until now i have managed to get by with google searches but my latest problem i just cant seem to find a resolve.

I have a worksheet that i use to track progress of activities based on a tick "a", I currently have a code set up so that when i double click the 100% cell an "a" is inserted in the cell and i have a simple formula =IF(AA6="a","a","") to populate the 90% then 80% etc to save double clicking each increment, the reason for the double click is i update the progress on a large touch screen TV.

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the problem now is i have thousands of cells with the formula above and it takes a long time to calculate.

what i would like to achieve is when any percentage is double clicked the percentages up to that auto populates with an "a".

any help would be greatly appreciated.