Pivot Table calc field problem
 

Dear experts,
I need to insert a calculated field in a pivot table to perform a % of
another field. I do want it to be a calculated field rather than a field
setting because I need all my users to be able to drag it in and out the
table keeping it identical.
My problem is that I can't seem to reference the total values. In a
simplified example:

Quarter % Neg Volume
Q1 50

looks OK, but when I add details to the negative volume it looks like

Quarter % Neg Volume
under 10% between 10 and 20% Grand Total
Q1 100 100
50

Rather than 17 33
50

My formula is "Negative Volume"/"Total Volume"*100

Does anybody have a solution to have the Pivot Table execute the calculation
I am looking for?
Many thanks in advance.
Best regards,

Valeria