There are a lot of variables, including your casting style and distance. I no longer write the line weight on my bamboo rods, since you have to experiment under actual fishing conditions to find what line works best for you.
The best example of this was an 8-1/2' rod I made a few years ago. I used stress curve analysis to make sure the rod would work with a WF6, and confirmed this with a deflection test similar to the common cents system, plus an oscillation test developed by Dawn Holbrook. The rod did cast and fished well with a WF6, but one day I forgot the reel with 6wt line, and had no choice but to fish with a WF7. It was amazing how much better the rod performed with the WF7! I was beach fishing with little back-cast room and larger flies, and the WF7 gave me more leverage to get the distance I needed and turn over the leader.
So, my suggestion is to take your rod to the local fly shop, ask for their advice, and try some different lines.