nubo,
As long as you don't have an overly long or heavy sight adding a forward pull on that bow there is no need for any back-weight to acheive a good balance. The Patriot Dually is balanced very nicely and neutral, and nothing like the 2002 Patriot SC which really demanded back weight.
An 8" by itself will be fine. I was shooting mine with a Spot-Hogg 'Hogg-it' sight on an extension bar and I still liked the balance of the 8"