I have rows from a query such as:

Name                  Type            Quantity

Customer01       A N              5
Customer01       A P               4
Customer01       O N              3
Customer01       O P               2
Customer02       A N              3
Customer02       A P               9
Customer02       O N              5
Customer02       O P               7

So, I can do a grouping and get the total for each customer:  (using sum(Fields!Quantity.Value))

Customer01     14
Customer02     24

What I needed is on the report to then show what percentage of the total each line represented.

I finally found that the sum function has a scope you can define to do this.
So I ended up with an expression added to a new column out to the side of the Quantity column on the report that looked like this:


The “table1_group_code” is the name of the grouping where I needed the percentage calculated from.

You can find this name in the Row Groups area of the reports designer.