Some express chain wear as a percentage and others as a mm? Lots on utube express chain wear as a % only my view but think most wear tools measure as a mm but again on utube its said as a %?
From your link, David
"It’s often stated that a worn chain is when it reaches one percent growth from the original 0.5in (12.7mm) pitch."
and
"So therefore anything past 12, 1/16 inches (0.5 percent) would be the time to replace a chain. And anything past 12, 1/8inches (one percent) has been worn to death and so a new cassette is likely needed."
But I see what you mean about managing to confuse something which is straight forward.
A 'tool' can only measure a distance by proxy: in this case the 0.75% 'tooth' drops through showing that the 5 links: 5" (127mm) has increased to (at least)128mm. I hoped surprise would be inferred when
@wafter said their chain was 2-3% elongated "I reckon my chain's only at about 2-3% wear so I'm expecting 4-5k from it" and I'd expected them to have experienced slipping on the cassette well before now.
I am running on my current (9 speed) cassette and chain together (currently @ ~4700km), rather than replacing the chain when it gets to 0.6% and getting 3 chains to one cassette. But the latter routine is far more economic with 10sp and particularly 11sp cassettes.