Numbers Pivot table, Grand Total as Average

I am using v11.2, which includes the new pivot table capabilities (most current version at the time of posting).

I have a sheet with all my bank transactions from a CSV download. I have added a "Category" field to this sheet, for every transaction.

Selecting all the transactions, I create a pivot table with the following config:

• Columns = Date (YM), so it goes left to right per month
• Rows = Category, so it lists each tagged category from top to bottom
• Values = Amount (Sum), so for each month it sums each category

By default, it adds a "Grand Total" column and row at the end. I would like to change this to calculate the average for that row (Category) not sum. Is this possible?

The only option I have seen is to show or hide the grand total. In Excel, I think you can right-click the column and choose the calculation to be used but it doesn't seem to be here in Numbers.

Or is it possible to add in an extra "calculated field" to do the same thing?

Thanks