Adding a column to a pivot table

Is it possible to add an additional column in a pivottable that has a formula based on data in the pivottable and that will get refreshed as the pivot table data gets updated.

For example in the attached picture, I want to make a column after "grand total" that has sum of "a" and "b" as a percentage of total, sum of "a" and "c" as a percentage of total and various other combinations.

I have tried just adding a regular formula, but this does not get updated as the amount of data changes.
Any help would be greatly appreciated.
Thanks