Interesting discussion on the 'Mother Board' on 'under lining/over lining' a fly rod. Unlike 2hander lines that are all over the map, single handers just count the first 30 feet. A WF '4' will be just that with very little extra being added via the shooting backing (some but it takes a lot to make much of a difference).

Ah, but Double Tapers? At 30 feet you've got a '4' but for each 6 feet beyond that you've gone up (effectively) one whole line size. I knew the line (in total grains) would 'grow,' but I had no clue how little it would take to make a significant difference.