The nature of bamboo is such that a number of weight lines within reason may be used. Depending upon the feel and skill of the caster, you may find a line one weight either higher or lower than the designed line weight may suit your casting better. I have several boo rods and I find that a different size line and shape (WF/DT) than the designed line weight makes a difference in my casting. I have a designed 4 weight boo rod that I find I can better handle with a 5 DT line. Suggest that you try several lines and determine what is best for you.