I have only been fly fishing for about 3/4 weeks and would really like someone to explain exactly how to pick the proper leader . The local fly shop guy always tells me what I need (4X 9 FT 5.4 LBS) .I just was wondering what does 4 x mean and how to I determine the proper length?
I see on the back of the package it shows a chart with fly size...how do I determine fly size?

My rod is a 8'6 St Croix 6/7wt