I am new-ish to SSRS (2012) coming from a loooong background using BusinessObjects, so it is sometimes hard for me to know how to search for the correct terminology. 

I have a report like this, where the sections are in on tablix and the Grand Total in another. 
This layout is required (or this would be a bit simpler!)

```
Section 1
                Date 1  Date 2  Date 3  % of Grand Total
Product1        value   value   Value   Value3/GT3 (12%)
Product2        value   value   Value   Value3/GT3 (14%)
Section Total   Total1  Total   Total3  Value3/GT3 (7%)

Section 2
                Date 1  Date 2  Date 3
Product1        value   value   Value   Value3/GT3 (30%)
Product2        value   value   Value   Value3/GT3 (22%)
Section Total   Total1  Total 2 Total3  Value3/GT3 (22%)

Grand Total     GT1     GT2     GT3     GT3/GT3 (100%)

```

I am having difficulty calculating % of Grand Total, as the numerator and denominator are not in the same scope. 
I thought maybe a Report Variable, but I can't figure out how to create the equivalent of this kind of formula from BusinessObjects - 

```
=Sum(Value) where (Date = "Date 3")

```

I want to assign the context or scope for my report variable but I don't know how to do that, or if it will even work.

I know I can create the scope of the whole report by including the Dataset name in quotes, but how to limit to a subset of that? 

Thanks, 
nod.