It's difficult to test since you don't have any data that matches your upload dates, but you should be able to use set analysis to do the same thing as the if() but with higher performance. Perhaps something like this:
Thats a good point...I've attached another version of the test QVW as I can't get the set analysis to work.
Is it worth stating that I need to the formula to work using the month and year of the upload date...there is a field called UploadMthYr which is basically a concatenation. I think I need that to link to the MonthYear field within the MasterCalendar