I understand that the tippet is picked by diameter. But wouldn't Rio call 3x tippet 6 lb when in reality it is 4.9 lb or something (I am guessing here). I understand that it is .oo8, and that is what a flyfisher looks at, but would they not also benefit from boasting a higher test than say, Umpqua?
So I think that overstating test is a given, though I have never tested them. The point I was making is that the comparable tests are vastly different diameters. If you take the labels at their word, and I contend that tippets have just as much reason to lie to be considered stronger than the others (think Frog Hair), so X value is the comparable based on test strength based on the those labels.