Office XP SP2

I am trying to add a new comparison field to my existing pivot table, but am
willing to consider a different approach entirely if you have a brainstorm.

My pivot table has time periods for columns (P4 05, P5 05...P4 06) with
Units and Dollars as "sub-columns" for the data field. Each month, I paste
the latest month's data to the end of the existing data and refresh the table.

Now that I have a full year of data, I need to compare each period against
its year ago period (i.e. P4 05 vs. P4 06) to determine the % change
(Formula: (P4 06 - P4 05)/P4 05). I just can't see a way to insert a
calculated field that can compare the 2 separate columns.

And just to make this really fun, I need the subtotals for the rows to
calculate correctly and allow filtering by the 4 page fields I have set up.

I hope this is a fun brain teaser for someone out there! Thanks for
suggestions!