aracer, your calculations are fine, but the point is that once it’s got to 1% and lunched the cassette, it will continue to work for ages. People are riding around on cassette/chain combos that are a year old easily. It’s a noisy mess at this point, it may be inefficient (not sure) but it’s not really at any greater risk of breaking so why not? It doesn’t do your chainrings any favours though.
I don’t measure chain wear, I look at the chain going over the cassette. A new chain binds on all the teeth as it goes round, as it starts to wear it binds on fewer. I like to change when it gets down to about 4 teeth before you can see a gap.
I once bought a bike that had a ti XTR cassette, that’s when I started to really keep an eye on the chain. The cassette lasted 18 months of lots of riding, and still wasn’t slipping when I changed it. It just got really noisy and rough after one change.