RDLC subtotals

Hi all, hope some1 can help. This is the expected in old NAV version:


And this is current RDLC layout I have. Im trying to make subtotals for each fixed asset so I groupped by autoincremental field (first column in table, lfdnr) so that way it shows me all the rows calculated in the process that builds this up. I would need to subtotal for each fixed asset (Inv-Nr. in the screen capture).
Groupping by "document no." Would be OK I guess too, if this make it easier.

Thanks a lot for your help

Any ideas how I could do it? I tried several ways but...