Calculated Fields within Pivots

Hi,.

i am new to this so please excuse if this is incorrect way to ask for help.

I am using a pivot table with financial data inc Sales, Costs, Margin % and % of total

I need to add a Calculated field which will allow A calculation using each row of the % of total Vs the grand total of margin %.

Sum is % of total*(line items Margin % - Company Margin %). This will allow for a Margin Impact % to be calculated.

As the % of total is purely a measure of sales % of each row and not a caculated field i cannot see the % of total to use in the forula of the new Cacl field.

sorry if this is not clear quite hard to put this into writing.

Regards
Simon S