Calculating percentage within same scope

Hi,

I have a report with a grouping on the GL-account-category and simply shows the sum(amount) for that category.
One of the categories is "Net Sales".
What I would like to achieve is the sum(amount) for all other individual categories as a percentage of the sum(amount) of category "Net Sales". The point is that all categories reside in the SAME grouping (scope).

So, as an example, the percentages that I would like to have:
Category "Net Sales": 100% (= 100k)
Category 2: 60% (asuming that sum(amount) of this category is 60k; divided by 100k = 60%)
Category 3: 25% (sum(amount) = 25k; divided by 100k = 25%)
Category 4: 5% (sum(amount) = 5k; divided by 100k = 5%)
etc. etc.

Hopefully someone can help me out.

Thanks!