I'm glad I re-visited this thread because I forgot to buy the line weight scale and can add it to my Christmas list.
OK, I just did a test with my old 12 Sage RPLX which is a very stout stick - absolutely nothing like the 9 wt Sage RPLX. I have heard that they re-named this stick as a 13 wt in the RPLXI series but that is just rumor.
I tried a Sci Anglers 15 wt billfish line on it first out in the field without a fly or fluff and measured from the end of the fly line to where I held it in my hand for easy pickups, good tight loop size, and enough line out to be able to shoot well, or to false cast without a big strain (so long as it didn't go on for too long
Here is the picture of the line taper.
With the above Levethian line the sweet spot for me with that rod, casting comfortably with tight loops well under control, either false casting or shooting (and not going for excepional distance) I was holding the line in my line hand at :
Subtract about 8 feet for the rod length above my line hand and that leaves an overhang of about 15 feet.
I then put on a Rio tarpon taper 12 wt floating line on, pictured below.
With the above floating 12 wt using the same criteria as with the Leviatan my hand was holding the line at:
66.42 feet minus 8' leave an approximate overhang of 19 feet.
Just for grins and kind of a self check. I then put on a Rio 12 wt tarpon in a sink tip line and the result then was;
66.08 feet . So I believe that my rod loading feel was pretty consistent.
Now, I cannot cast the Hardy Sintrex with that Rio 12 wt without shortening up my carry way more than what I am used to. I would have to use a maximum of an 11 wt in that Rio line with the Sintrex to cast in my preferred manner, but it cast the Cortland 12 wt Crystal PE just fine - without a big fly along.
I have some big popper heads coming from Cam Sigler and am thinking that the 15 wt will work well with them on the 12 wt rod. They are 1 1/2" in diameter and very wind resistant, of course. The 12 wt Rio, rear weighted as it is, is a lot of work just getting out a regular sailfish streamer. More weight is needed up front to cast well with it when using big flies
Though a 10 wt rod is more than sufficient to fight east coast sails, it is not possible to throw big poppers with one.
So my points in this post are several.
1) Don't get too hung up on what line fits what rod. It all depends on the caster, and what he's casting.
2) Notice the incredible taper differences between those two lines. They are exact opposites!
3) You can vary the actual line weight (that's loading the rod) considerably by simply changing taper lengths with the same line wt ratings.
4) To elaborate on #3, you do this by extending the carry (particularly with long belly tapers or rear weighted compound tapers like the Rio Tarpon line)
5) As someone else mentioned, size the line to the fly or popper.
6) Experiment! Especially with friends lines so you don't have to buy them first.
PS: I realize many previous posts have the same information. I've simply said them in my own words and added a couple pictures. IMPORTANT!! When comparing line tapers you HAVE to compare only the floating line tapers when buying a sink tip. The sink tip section is much more dense than the floating section in the head and will give an erroneous picture of weight distribution.