Using a Pivot Table total in a calculated field

[miafs]

Help!!
I have a pivot table field as follows:
one column shows company sales by product
one column shows market sales by product
one column is a calculated field that shows company sales / market sales (market share)
Each column has a grand total

I want to add another column that shows market sales * "average market share"

How can I reference the Grand total of market share in another calculated field. I have tried using getpivotdata in the calc field but it doesn't seem to be allowed.

Thanks in advance for any help

 

[BrianB]

Cannot use PT totals in calculated fields in a PT.

Might be able to use a =VLOOKUP("Gross Total", ..................