To open in a new window I use this

="void(window.open('http://ServerName/ReportServer/Pages/ReportViewer.aspx?/Path/ReportName

&ParameterName=" & Parameters!ParameterNameDepartment.Value & "

&rs:Command=Render&rc:Parameters=false&rc:Toolbar=false', 'WindowTargetname','width=1025,height=600,location=no,toolbar=no,resizable=1,scrollbars=yes,dependent=yes'))"

It should be all one line but I CRLF'd it to make it more readable. For the url I usually open the report I want to jump to and just copy & paste the URL. I added a papameter too so you can see how that part works. yOU can name the wondow too by replaceing the Window Target Name Value.

This syntax is very picky, a lot of times I have to cut my losses and start over with one that workss and slowly replace stuff until it works.