Summing % Difference from in a Pivot Table

I have revenue data which ranges 14 months and represents 1 full financial year (FY) and 2 months of the current FY.

Using a pivot table I have compared the '% difference from' FY08 to FY07 on a month by month basis. What I am having trouble with is seeing the year to date '% difference from'.

This should show at the end of the pivot table the '% difference from' FY08 to FY07 for the first 2 months.

I have attached a copy of the fields I want the pivot table to populated.

Please help.

Thanks