.75 means replace chain very very soon
1.0 means replace chain now or it may be too late
I disagree. .75 is change chain now. 1% elongation is usually change drivetrain though you can sometimes get away with keeping some parts if your lucky.
I’ll put a different spin on it.
I let one of my 9 speed XTR drivetrains wear a little too much but decided to change to drivetrain when it starts to play up. That was three years and ~5000km ago and it’s still working beautifully.
So all is not lost if you let things wear a bit past the simple chain change. Leave the chain on and change this when they are completely worn out.
It’s also worth saving the worn chains, chain rings and cassettes as most of the time they have plenty of life left for commuter or occasional bike builds.