calculated item does not work in grand total column in Pivot
 

The calculated item I am using for a percentage change works in all cells but
does not compute in the "Grand Total" Column. The formula is "$ diff 07 to
06/2006" where the numerator is also a calculated item.
Putting the cursor in the Grand Total cells and then going to "formulas"
does not give me the option to enter a new formula.
Any help would be greatly appreciated!