Jack - It would be good if you can post QV document with some sample data. I am sure one of us will guide you. The idea is to build a robust data model to accept data month on month and code re-usability.
Just to give you some ideas... here are few points to consider.
1. Calculate the CurrentYear & PreviousYear by variables (Something like this...)
CurrentYear : Max({1} Year)
PreviousYear : Max({1} Year) - 1
2. Calculate CurrentMonth by variables (See below...)
CurrentMonth : Max({1<Year = {$(=$(vMaxYear))}>} MonthNumber)
3. Use the variables in Set Analysis
Sum({1<Year={$(=($(vPreviousYear)))}, MonthNumber={$(=($(vCurrentMonthNumber)))}>} Sales))
I hope this helps!
Cheers.
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