Adding field to calculate sum of a field and count of a field in pivot table

Hi all

I have set up a pivot table, which shows the count of one field and the sum of another. My aim is to add a further column which will divide one by the other. I have tried to do this using the calculation field function, but it only allows me to divide the original fields, not the count and sum I have created in the values area of the table.

I have attached the table below. I would like the extra field to divide sum of runners by count of races.

Any help would be greatly appreciated.

Gemma