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@Kimmo
Chain checkers like that aren't super accurate anyway, because they have to include a fudge factor for roller slop.
To measure actual elongation, hold the chain tight with the crank and hold a ruler with the 0 at the centre of a pin, and look at 254mm. How far away from 254mm is the next pin? Work out how many more percent it is, say 255.5/254 = 100.59%.
If it's more than half a percent longer, change the chain. If it's more than 0.75% longer, you probably need a new cassette too. If you let it get too far, it'll then start to chew up your rings.