You're making it too hard
Decided to switch from using scandinavian heads to more traditional lines. Matching such line in terms of grains to particular rod is still a mystery for me. When compared to skagit do I want same, less, or more grains to balance the rod with short to mid belly line? Lets take a rod that casts 440 grain skagit. How many grains a short belly should I aim for?
Most longer lines (e.g., NextCast, Beulah Aero, Ballistic Vector, Delta, ...) are labeled with two rod weights. You have a 7-wt rod, get either a 6/7 or 7/8 line. In most cases both will work, depends whether you want a lighter or heavier load. NextCast tends to run heavy, when in doubt I match the high number, that is, 7/8 on an 8-wt rod. Carrons are their own creatures, it's almost line by line. One reason for this is in longer lines the distribution of weight matters at least as much as the total weight. For a skagit belly at 23', given it is also a necessary design consideration the business end be thick enough to turn over heavy tips, there is little room for design variation. Most such lines will cast similarly at a given grain weight. In a 60' head the weight distribution matters an awful lot resulting in lines whose performance is much more design dependent. Carron will not even tell you the grain weights.
That said, for a head in the 45-55' range, like an Aero or NextCast 55, you might notice the above rule usually lands you close in grain weight to a typical skagit belly for a given weight class. If you like a 510gr skagit on a given rod, get the Aero 7/8 or the NextCast 6/7.
As already mentioned above, the Meiser and Rio sites are good resources. Even if you don't intend to buy a Rio line or Meiser rod you can learn a lot looking at the data for different rods.