How do I create a calculated field in a pivot table that simply divides two other columns in the pivot table?

I have these data in Excel:

I created a pivot table that is supposed to calculate the percent of each state’s counties that qualify for a certain government program (this is artificial data, though).

I want to create a third column (field?) in the pivot table that simply divides the second column into the first column to create the percent of counties in the state that qualify. I can do this by creating a formula next to the pivot table, but then when I filter or otherwise change the pivot table, that will break.

Is it possible to do this with a calculated field? Creating a calculated field with = 'Count of County ID' / 'Sum of County Qualifies?' or ='County ID' / 'County Qualifies?' in the formula field throws an unhelpful error.

I want to use a calculated field here so that when I filter/sort/etc. the pivot table, the calculated field updates too.