Pivot Table % Difference Running Total

Hi,
I want to set up a pivot table in the format below with running totals by month.

Untitled.jpg

Is there a way to be able to get the % difference of a running total?
i.e. for Aug I have only been able to get it to show 40% for ProductA based on the monthly sales difference, but I want it to show 14% as per the above table!

Thanks in advance,

Stephen

I think you may be right. What I need to be able to do is a calculation on a running total within a pivot table.
I have attached an example of what I am after...

Example.xlsx

I have found a work around by adding a YTD column to the source data removing the need to show the data as a running total, but I would be interested if you have a solution.

Thanks