Relative numbers with drill down

[mrmoster]

I have created a chart where I have a drill down and I have used relative numbers to show the value as the proportion of total per a month. Ideally I would like the proportion when going down a level on the drill down to be of the top level total not the drill down level.

E.g. 

Category (Top Level)	Sub Category (2nd Level)	Sales
Tech	Item 1	10
Tech	Item 2	40
Clothes	Item 3	50

So I would want item on 1 to show as 10% of total sales and not 25% of sub category level.

Is this possible? 

[Or]

Sum(Sales) / Sum ({< Category = >} total Sales) should hopefully do the trick