Well i dont think its that easy .i use a rio afs 9/10 at 38g and a g2 9/10 at 42 g, they are both 9/10 but the weight is distributed differently in them, they both work very well on my 9# rod yet are 4g different
Ok, so if you were to do away with the labelling #9/10 on both lines so you have a 38g and a 42g line like you say they both work on your rod for you but I might not get on with them, I might like for example a 35gram line. My point is if I wanted to buy a line I could look for the weight in grams as a starting point to try a line instead of trying your luck with a line in the #9/10 window. I do agree its not that simple and nothing is better than spending time with a good casting instructor. But for me when I changed to weighing my shooting heads and then using the Zpey heads my casting improved. All my heads including my Skagit heads weigh in the window marked on the rods. Some have different taper profiles like yours do but I don't notice a big of difference in the way they work TBH.