Esa, nice work buddy! Your method is the rabbit hole I was talking about in my last reply to coalbe's post! If you combine the optimum rear half weight (for proper rod load), and if you know a certain length rod is most comfortable to cast with a certain head length to rod length ratio (this ratio is what I was asking Ard about), in theory this should reduce the guesswork significantly when choosing a line to match any given rod.
The only other contributing factors would be caster preference (does the caster prefer a deep or a shallow rod load), caster skill (primarily determines maximum head length), fly size/weight/air resistance (also determines head length), and weather conditions (wind, especially when the rod in question is a 3 weight trout spey rod).
I've already used this method to choose lines for my 6wt switch rod. It worked in that instance.
My thinking is that the heavier the spey rod rating (think 10wt), more deviation from the optimum rear half of head weight is possible, but still load decently. The smaller the spey rod rating (think 3wt) the less deviation is possible.
Example: I just took two SH rated WF6F lines out to lawn cast with a grass leader. One has a 36' head including a long rear taper, the other is 39' including a shorter rear taper. The short one was awesome! Easy casting, good speed, nice loop shape, shot a small amount of running line. Unfortunately I have no idea who made it, it came on a used reel I bought on an auction site. The longer one was awful. It required much more effort, speed wasn't as good, tailing loops were happening (so this WF6F is overloading the rod), no shooting line. I will be taking these to the water to test cast again. I lent my grain scale to friend, but when it's returned I'll be weighing both lines.
I forgot one more variable: Is the caster wanting a double taper line? This matters (especially on light spey rods) because the line still in the rod guides contributes to rod load without actually being cast.
Last edited by Captcaveman; 04-19-2019 at 07:42 AM.