Calculated Field in Pivot Table

Hello.

I'm using Excel 2007.

I have a pivot table that returns the data below. Note that Failure Rate is
a calculated field based off of Failures and Uses

Failures Uses Failure Rate
Category A 5 10 50%
Category B 10 10 100%
Category C 15 20 75%
Grand Total 30 40 75%

My Question:
Is it possible to create another calculated field that divides Failures by
the Uses Grand Total. So, something like:


Failures Uses Failure Rate Failures/Total Uses
Category A 5 10 50% 12.5%
Category B 10 10 100% 25%
Category C 15 20 75% 37.5%
Grand Total 30 40 75% 75%

Here's the math for the new calculated field:
Category A: 5/40
Category B: 10/40
Category C: 15/40

The piece I'm not sure can be done is to reference the Uses Grand Total in a
calculated field somehow. What I have to do now is get the Pivot as far
along as I can then create formulas off to the side. I want them to be part
of the pivot.

Any help is appreciated.

Thanks,
Jeff