Aha. Your first post didn't refer to percentages. I have not found a way to get Excel pivot tables to do what you want. It offers percentages, or running totals, but not running totals of percentages. The way I would do this is to simply add another column outside the pivot table that has formulas to do a running total. However, if new categories appear, that column will have to be extended manually (unless VBA comes into play). The formulas wouldn't have to change, just be copied down to new rows.
My specialty is "not elegant, but easy and it works."

Bookmarks