Calculated Fields in Pivot Tables using Existing aggregates

In an Excel [2003] pivot table, I need to calculate a new field based on the
COUNT of one field and the SUM of another (and other similar calculations).
Take the following example:

Date
1/1      1/2      1/3      Total
CountOfCustomers    10      20     15         45
SumOfBigCustomers  5        10    10         25
% Big Customers      50%    50%  66.7%   55.6%

How would I create the "% Big Customers" field? I know in this example it
would be easy since I am dividing by the total # of customers and I could
just take the AvgofSumOfBigCustomers (5 divided by the number of records in
the set 10 would produce an average of .5 or 50%). But what if it were two
fields that would not permit me to use the built-in summaries? In other words,
how do I use two different SUMMARIZED fields in a formula to calculate a new
field (i.e. =SumOfBigCustomers / CountOfCustomers)?

Let me know if this does not make sense and I will try to provide a better
example.

Thanks.