Common Sense
Common Sense tells me the Common Sense System is fault in that it assumes all rods will derive their maximum power at 1/3 their total length. Were that true, all rods would have the same action. No fast rods. No slow rods. And we know for a fact that is not true. It also dictates line weight based on these false assumptions.
Many moons ago, when the choices were DT, Wind Cutter, or Accelerator, and the line designations made even less sense than today, I asked Jim Vincent why an Accelerator line for any given rod was so much heavier than a Wind Cutter line for the same rod. His answer was that in order to lift and cast the longer belly line, you needed to extract more power from further down in the blank. Be that as it may,,,,Even then, we had the 9140-4 & the 9140-3, two very different rod actions.
Fast forward to 2014. Not only do we have a plethora of rod actions from which to choose, lines and heads out the ying yang, but different casting styles as well. There still seems to be more than a bit of confusion concerning line weights when it come to Scandi vs. Skagit. Scandi casting is a dynamic cast based on the kiss & go anchor system. Where as Skagit is based on the sustained anchor system.
The Scandi cast utilizes the weight of the entire head to load the rod sufficient to cast the line & fly. The Skagit cast, however, discounts the weight of the sink tip, it being stagnant in the water. The Skagit head (let's assume 2X rod length) needs to develop enough energy to load the rod sufficient to rip the sink tip (assume 1X rod length) plus fly out of the water & send it on it's way.
The rod I use has a grain weight (determined by me) of 400/750. The 400 gr head utilizes a 13 ft tapered mono leader, smallish fly and kiss & go type (Scandi) cast. The 750 grain setup consists of a 540 grain Skagit head + a 210gr MOW tip and sustained anchor (Skagit) casts. These are special purpose line systems which I reserve for extreme conditions. I also have a Skangit setup that is somewhere in between.
I started out casting a 450 gr head on this rod and went from there. As I learned more about lines and casting styles, my own common sense system began to emerge.