Averaging a column of calculated fields in pivot table

Hi,

Wondering if there is a way to display the average of values of a column in the Grand Total row.

Doing this is no problem if the values are directly from a database that one is trying to create a pivot table for but here;s the problem: my values in the column are all calculated and when I right click and try to change field setting all options appear disabled.

Will appreciate any help

Thanks