Pivot Table - Values as a % of Maximum of Parent Grouping

[Mayanwolfe]

I know that title sounds a bit confusing. I have various fees grouped by departments. Customers may only belong to one department, but they may pay one or more fees in their department. I have revenue by fee type, as well as the count of customers that paid each fee type. I would like to determine the % of customers that pay them within each department. The challenge is that since a customer may pay multiple fees, their records are duplicated, so I cannot use the % of Parent Grouping Total option. I would like to create a calculated field that divides the customer count for each fee type by the max customer count for that department. However, I've not been able to find a method that will give me what I'm looking for. See an example below. The far right column is the field I'm trying to create:

I can do this easily with formulas off to the right of the table, but I would like the option to change demographic filters on the pivot table, which will change the row counts for each department and mess up the formulas. Instead, a calculated field would shift appropriately with the table. Thanks for any ideas!