YTD Sum on a Pivot Table

I've been trying for a while now to write a formula that will allow to me to a YTD sum calculation on entries in a pivot table.

In this example, I have weeks at the top in the pivot table and I want to be able to calculate YTD sales for each retailer by category. The Dashboard tab is what the user will see and is for Retailer 1, and they can change the week they are looking at using the drop-down in cell H2. I need a formula for F9 that will calculate the YTD sales based on the week chosen (18th Jan), and the data given in the Pivot tab (it's a pivot table in my file, I just copy-pasted values so I could change labels. Also as the weeks only go up until the end of January each year, this will increase every week as more data is added.

As I am adding more retailers on a regular basis, simply manually entering the formula is not really an option since the position of each retailer in the table is likely to change.

Anyone know if this is doable?