Pivot table - Percentage of totals

I have written a report that will summarise the results in a pivot table. The report is filtered to provide all closed opportunities for a specific period of time. This report then summarises the data by owner, type, etc.

There are 3 columns for total opportunities (Total Closed, Total Won, Total Withdrawn). A case statement is cased to provide the totals for each column.

The pivot table displays the results, e.g.:

                      Total Lost     Total Won     Total Withdrawn   Strike Rate

Owner 1               10                  20                         5                          %

Owner 2                 15               30                          7                          %

I need to calculate strike rate as the % of total won against the total won and total lost, but not withdrawn. In example for Owner 1, this would be 20 / (10+20).