changing a calculation in a pivot table
 

I have a pivot table from a db that shows 10 rows of dates and data.
each week i delete the oldest row and add it adds the new one. What i
need to do is calculate the % change from the oldest to the newest
each week. right now i do it each time i run the db. what i want is to
have a cell calculate it for me. when i try that, i get an error when
i change the data.

looks like this
Date Data
12/8/2009 120
12/15/2009 100
12/21/2009 90
12/29/2009 92
1/20/2010 67
1/25/2010 54
2/1/2010 73
2/9/2010 64
2/17/2010 72
2/22/2010 74