That's a great way to do it. They state what the weight of a penny is in the Common Cents System so if you have a really accurate scale it would be a snap. It's kind of a bummer that almost all rods are not what they are labeled as. It's all in the marketing for what they want us to think they are and what it will do. Like the rod mentioned you were inspired by to make this thread. A 4 wt. with a 6.25 ERN. Everyone knows the bigger the rod, all things else being equal, the farther you can cast. So what was the idea most likely with the builder, and some will say it right in the ads, "Casts farther than the competitions rods". Sage was, and probably still is, one of the ones most guilty of this type of underrating their rods. You will never find a distance competition 5 wt. rod that's an actual 5 wt. rod. You can stamp any rod you want as a 5 and use it in competition, or at least before the PC police came in and protected everyones feelings. I saw an advertisement for a line where they said right in the ad "Our line is a full weight (not sure which way) than the industry standard. How can you say there is a standard at all when nobody adheres to it? It's kind of the definition of standard isn't it?
If everyone cast as well as my friend Bill and took the time to find out the ERN of the rods they have, then understood head configuration and weight, then everyone would have an easy time setting up rods for the way they cast. But everything is made to market it. In I believe, the mistaken rush to make something look like it does something better than the other guys, they make it harder for a good segment of the market and make them not like the products.