The old yoz-ami (YGK) website used to have several pages in English that was pretty through explaining world wide line rating methods. The best I ever found on the internet. The Japanese measure line, or rate line differently than it is done in Europe, which is different from the way it is done in America. The European system was kind of half way between the American and Japanese method. But unfortunately I can no longer point you to the yoz-ami pages as they took it down a few years ago.
The general thumb rule is x4.
2号×4＝8lbs ; 5号×4＝20lbs.
But it really depends on which lb. rating you are seeing on the spool of line. lbT or lbC:
American made line generally uses lb.-test (lbT) which means the line is guaranteed not to break below that rating. iow, 8 lbT line will not break below 8 lb. It also might not break at 12 lb. and finally break at 15 lb.
The other lb rating is lb. Class (lbC) which is kind of the opposite of lbT.
8 lbC line is guaranteed to break at or below 8 lb. It might break at 6 lb. or 7 lb. But will definitely break at 8.1 lb.
Maybe the following website will help. Even with the odd translations. It has some explanations, plus tables for FC, Nylon and PE lines.
Google translation of https://ms-fishing-style.com/ozgglb/linestandard
btw & fwiw - years ago I did some research into the Japanese line 号 or 号数 rating system, trying to understand it. It appears that the only standard set by Toray ( I think in the 1960s or earlier) is for nylon line of 1 号 = .165mm diameter. Which was itself based on an ancient method of measuring silk thread. All other lines; FC, PE and lines of larger or smaller size are just kind of on the honor system to have other size lines be in compliance with their line size tables & that nylon reference size.
The line sizes are not ratios of the line diameter. They are ratios of either line cross section area or ratios of the line weigh.
[ it is difficult to measure small line diameters so lines are measured by denier weigh ( X grams/9000 m) during manufacture. If 9,000 m of line has the correct gram weight for say 3号line, then the average diameter of the line will be correct at 0.285 mm +/- manufacturing tolerance.
Or another way to understand the Japanese line system is 4 号 line will have 4x the cross section area of 1 号. (It’s easy math. pi * r^2. for 0.330 mm vs 0.165 mm line --> 4 号/1 号 = 0.08553/0.021382 = 4.000094) I can never type that formula with seeing either Jethro or Granny Clampett saying, “No, pie are round - cornbread are square.”
Anyway, 4 x X 号 seems to be common thumb rule.
But I think new chemical mixes for making the lines. Throws lines off the standard lb rating vs standard line diameter table.
Maybe this half-way answered your question.