A grain scale is a good thing to have, because individual lines differ in weight depending on taper.
The old green level lines from Rio
(ranging from ~ 12 to 22 grains per foot) and any of the heavy DT lines are increasingly hard to find, but are perfect for this sort of thing. But, heavily used lines can have plastic degradation that impedes welding.
Cutting up cheap eBay WF lines is a decent alternative. The main belly weights of WF lines are heavier than AFFTA 30’ specs, because AFFTA includes the forward taper. Some rough guidelines are below:
For T-14, use WF12 belly (~ 13.5 - 15 grains per foot) to turn over T-14
For T-11, use WF11 belly (~ 11.5 - 13 gr/ft)
For T-8, use WF9 belly
These are for standard fly lines. Again, every line taper’s belly weight is unique. More extreme tapers (e.g., overweight bass tapers, or fine Triangle Tapers) diverge predictably from these guidelines