Attended a local sportsman show on Saturday and a local fly line rep
was giving a fly casting demo. He made the point to move down one line
weight for every ten feet of line beyond the 30 feet. He also recommended
weight forward line and his demo sure proved his point.
It makes "cents" , just wandering if there is a rule of humb to back up his claim?
Any thoughts would be appreciated, much thanks, stan yerger

Yes there is, somewhat at least. But you have to assume that the rods he's working with are properly loaded with 30 feet of line past the tip. Many these days are not.

Read this for a better explanation.

[www.common-cents.info]

Then look under "Additional Information" and click on "AFTMA Fly Line Components" and I think it will all make perfect sense to you, or anyone. This is how it was supposed to work from the very beginning.

...............

Fly line weights are determined by the first 30 feet of line. If you're carrying 40 feet of line IN THE AIR then the 10 additional feet might actually mean that the line is heavier than the rod is rated for. So then it would be appropriate to drop down a line size so that the weight of the 40 feet of line matches up with the line rating of the rod.

At least that's the theory. In practice, you probably wouldn't be carrying 40 feet of line in the air, you'd be false casting 30 feet and then shooting the line to gain additional distance. That being said, if you're continually casting either very short or very long, there's nothing wrong with going up or down a line size.

Weight forward lines are just plain easier to cast but if all your casting is short range, there's nothing wrong with a double tapered line. With the double taper, you're getting two lines for the price of one. When the front side wears out, just turn it over and use the back side.