I've bought a chain wear checker made by BBB, but not sure what the percentage of worn is about?

It has 2 sides, one of them indicates that the chain is worn 75% and I should replace it in the near future. The other side shows, that the chain is 100% used and should replace it as soon as possible.

My detailed question :

What does these percentage mean? What is the equivalent of it in the metric system? 

Will 75% worn state affect my cogwheels also? Can I replace the chain to a new one without changing the drivetrain's other elements? 

I use a single-speed bike. Chain width is 1/8 inch. It consists 94 chain. (400km (250miles) / month)


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