Using 'The count of' column in a pivot table formula
 

Excel 2007. In my underlying data I have 2 columns. Col A = Invoice numbers.
Col B = Product Sales in $. One invoice number may apear many times.
In my pivot table (that points to my underlying data) I am grouping the
count of unique invoices in Col A and the total sales value associated with
each in Col B. I now want to add a formual in my pivot to calculate the
average $ value per invoice = 'B/The count of A'. My problem is I can't seem
to refer to 'The count of A' in my formual. Any ideas??
Many thanks.
--
Peter