Force Calculated Field to occur at a given level of a hierarchy and then roll-up

I am trying to create a calculation that is ALWAYS done at a particular level of the product hierarchy (i.e. Product Category) and then rolled up to higher levels of the product hierarchy.

Assume the following product hierarchy:

Product Type -> Product Group -> Product Category -> SKU

I want to 'force' the following formula to always occur at the Product Category level:

(Qty (current period) - Qty (last period)) * Avg Margin Dollars

...then roll up the answer (which is dollars) up the product hierarchy together with the rest of the Categories to Product Group and Product Type.

I can easily define the formula that I want but I cannot force the Calculated Field to always start at Category then be rolled up.

Any ideas?