You can rate a rod; we already have a good barometer on that. And we already have a good system to rate a line for said rated rod.I'm not buying this "there are too many variables" stuff, it's woo. If that was actually true it would be impossible to rate a rod at all and every angler would have a completely different opinion about a rod. That does not seem to be the way it is.
Now, you just need a rating for anglers, for the rated rod and line combo - to ensure it works for him/her.
Angler rating #1 = crap caster
all the way to #10 = excellent caster...