How can I summarize the result of Lookup values?
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I've a SSRS Report where one column is a result of Lookup from another dataset.
I need to show the total of this column, 
For each row I'm using the following expression
=Lookup(Fields!InstallerID.Value, Fields!InstallerID.Value, Fields!RoutedPoints.Value, "spCapacity_GetRoutedPoint")

and for a total row I'm using 
=Sum(Lookup(Fields!InstallerID.Value, Fields!InstallerID.Value, Fields!RoutedPoints.Value, "spCapacity_GetRoutedPoint"), "spBSA_National_Productivity") 

But the value is wrong!!!

How can I get the summarize of this column?
Thank in advance
Eliana