Calculated member in Grand Total area of cross tab

I'm using CR2008.

My cross tab contains one row & one column selection, but has 10 value lines within each row group.

One of my value lines is a % value - from the DB, not calculated.

In my grand total area, this value is summing rather than calculating the correct value, although the values the % is based on are both in the cross tab, so could be used.

Is any one able to assist with the required insertion, group & header formulae in order to insert a calculated member into my cross tab to show the correct value?

I have the formula working in the details section but am unable to get the member to insert where I want it to, or to calculate the correct value for each cross tab column!