weighted average % calculation in pivot table

Hi, this is I think a simple question but it is puzzling me.

I want to do a weighted average calculation in a pivot table. I have different product categories (call them A,B ,C etc). I have profit in an excel sheet for each product sold by customer and date. In the sheet I have a column calculated for margin % ((sales - cost)/sales) each time a product category is sold. In the Pivot table however, it appears that Excel only does a simple average of this column.

I thought I could solve the problem by using the actual margin (sales - cost) and then summing it in the pivot table and then dividing that by the total sales for each product category. I went to Value Field Settings, Show Value as and I tried to do it as a percentage of ... but I am not doing it correctly since I could not figure out how to use the sum of the sales value for A as the denominator or base item using the calculation tab. I have attached a simple sheet to show what I need. Thanks.EXAMPLE OF WEIGHT AVERAGE NEEDED.xlsx