Have you ever needed to create a new calculation or custom grouping in a pivot table? Try using the New Calculated Item option under the More Options button belonging to the metric/attribute on which the new calculation will be based.
You can then create a mathematical formula or other aggregate grouping for your calculated item using the Calculated Item screen. Give it a try, and test out your results. One quick warning: each calculated row CANNOT be moved up and down the list of calculated items. If you have created 25 new calculated items, but have forgotten to create an item that should be located at the 10th spot, then you will have to recreate ALL of the calculated items after #10.