With a number of Orvis bamboo fly rods in the rod racks, the rule of thumb for Orvis bamboo seems to be to fish one line size lighter. The #8 - 8' always has been fished with a WF 7 F or WF 7 ST unless throwing really big deer hair flys to bass. The #5/6 - 6 1/2' is usually fished with a WF-4 -F now, before a WF 5 F & when new the WF-6-F. While another 6 1/2' with a #5/6 line designation is so much better with a #3 DT or the WF-4-F. With an Orvis 8' superfine graphite rated for a #7, casting with a #5,6,7 or 8 weight flyline works with the rod; but the bamboo isn't quite that versatile unless you learned to cast with bamboo first & then it sometimes becomes more work than relaxation while fishing in getting the timing perfected for the distance you are generally casting; or if you are fishing within 45 feet go down a line size !