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

Hi,
as you posted a jpg I couldn't send you a working example
Do the follwoing ( this is with XL2003) :
open the pivot tabel toolbar
click on pivot table
select " formulas"
select " calculated fields"
follow the wizard to create the desired functions
adapt the way data is shown " %of, etc..) as usual.

You will also fond lots of info at :http://www.contextures.com/xlPivot10.html

thanks, but im not sure if that will apply to my example. I have attached the spreadsheet. The column next to grand total is what i am trying to achieve, but i cant work out what the formula would be if i were to create a new field.