code for adding a '% of total' column

What code does one need to add a single column to a pivot table for '% of
total'
As in the percentageOfGrandTotal, as shown here.

Product MonthA MonthB MonthC prodTotal % of total

prod 1 val1 val2 val3 valTotal
percentageOfGrandTotal
prod 2 val1 val2 val3 valTotal
percentageOfGrandTotal
prod 3 val1 val2 val3 valTotal
percentageOfGrandTotal

monthTotal valTotal valTotal valTotal grandTotal 100%

Any help, clues or pointers would be much appreciated.