Specifically the increase to damage dealt.

So I went to the dummies at the wolves den because I wanted to see if Straight Shot or Empyreal Arrow was better with Barrage. (They both have a pot of 1000 so it didn't matter.) But I found out that I'm doing the exact same amount of damage (with all weaponskills) with 4 stacks of repertoire as I do with none. Also, it's not really clear in the tooltip if I only get a flat 5% increase or if it's 5% for each rep. (20%) If it's a flat 5% then why do I need to stack it to 4? Any help would be appreciated.