How to Add a Percentage into a Crosstab

Hi,

Running XI 11.0.0.1282.

I have to summed fields in my crosstab: Payments and PreviousBalance. I need to calculate and display the percentage of payments.

I've gone into the Crosstab Expert and added both fields to the crosstab. When I highlight the Payments, Change Summary and select Show as percentage of; the only option to select is itself.

How do I get the Payments to show as a percentage of Balance?

Neither of these fields are formulas. They are both summed, not counts or averages, but I can't get the percentage to point to any other field.

Thanks,

Brian