I am trying to create a query that returns the best and worst performing products for a given customer, based on this year's net sales versus last year's net sales, weighted by the total net sales for all of the products sold to this customer in the last two years.

I have created Last Year Net Sales (up to the last month end): AGGREGATE(PERIODSTODATE([Invoice Date].[H1].[YEAR],[Invoice Date].[H1].[Month].[NOW-13]),measures.[Net Sales])