I have a report set up basically like this:

Group1
----Group2
--------Data
--------Data
----EndGroup2
----Group2
--------Data
--------Data
----EndGroup2
EndGroup1
<insert another Group1 here>

At then end of each 'Group2', I need to display the sum of all the items in that Group2.
At then end of each 'Group1', I need to display the sum of all the sums calculated at the end of each 'Group2'

Currently, I have just placed a summary field at the end of both Group1 and Group2. However, I'm not sure that this is the best way to do this, because then the Group1 sum is calculated by adding up all the data items again, even though it could just add up the Group2 sums, and I have a LOT of data items.

So is there a better way to set this up?

Thanks.