Creating 6 month moving average key figure?

Hi-

I have a power user who needs to generate a "complaint rate". He needs to be able to divide the number of service notifications received in a month by a 6 month moving average of units shipped. In this same query he needs to show the complaint rate for the past 6 months. I am not a query writer...I only know the backend. But I am at a loss of how to select 6 months of data upon a load to calculate the 6 month moving average of units shipped. Any suggestions/ideas would be greatly appreciated. (Or is this a calculation that can be created in the query?)

Thanks!

-Jenn