Calculate filter value against grand total

Hi guys,
Is it possible to calculate a filtered value as a percentage of the unfiltered value?
i.e. I want to see new sales revenue, (first purchase over previous 3 months), against all sales over previous 3 months.
I can see the grand total on the grid if I choose ‘Show Others’ but can’t find a method to reference it in a custom calculation.
Any help much appreciated.