Since you are on SSRS 2008R2, you can use the aggregate of aggregate functionality that was added in that version.
You were on the right track; you just need to add a Scope value to your expression.
I'm using a version of your data, and have constructed a simple tablix:
Note that I have created a group called Line.
To get the Sum
of the Max
HourTarget column, use the expression:
=Sum(Max(Fields!HourTarget.Value, "Line"))
This works out the Max
for each Line group, then takes the Sum
of these.
Now we have your required value:
Prior to SSRS 2008R2 there was no easy way to do this; typically one would add an extra column to the DataSet with the pre-aggregated value to display in the report.