After I watched this video https://www.youtube.com/watch?v=8LqTmvuf6tw I bought a tool like that.

While trying to maintain my bicycle I decided to measure the chain using this tool, but how do I know if I should use the 0.75 or 1.0 side of the tool? One side sits kind of like the "new chain" in the video (at 00:58 into the video), while the other sits like the "old chain". Since my chain is old, I'm pretty sure what side I should use, but is it possible to tell this for sure, i.e. what side should be used? Is it written somewhere on the chain itself? On the gears?

Edit: Also, the one in the video has a different range than mine, it is 0.5 to 0.75. How do I know if my tool is right for my chain? What is the unit for this? Inches?

If the 0.75% tab fits into the chain links, the chain is due for replacement. If the1.0% tab fits, the chain is overdue and you are very likely to have to replace the cassette or freewheel too. If neither fits, the chain is still ok to use.

https://www.pardo.net/bike/pic/fail-0...l#park-rohloff

shimano and now one other company make a tool that is an accurate chain measuring device. Not having one of those, a ruler is the most accurate way to measure chain wear. Replace a chain at 1/16" in 12" of chain.
The guy on the video is full of it.

The Pedro's Chain Checker Plus looks like it works similarly to the Shimano tools, i.e. by pushing both rollers in the same direction instead of away from each other.

When across 6-full-link the roller-to-roller distance is 132.6 measured with a good caliper...although that is rather conservative, you can push it to 132.7

When it reaches 11 1/16 or 12 1/16 inches measured with a ruler between the same point on the nearest pins. That's 0.52 - 0.56% of elongation which won't have worn the cogs enough to preclude mating with a new chain.

By the number or sooner , realizing preemptively replacing the chain extends the wear life of the cassettes.

If anyone is interested I decided to just replace the chain without replacing the cassette at the same time.

My experience so far is that there is no slipping on the gears whatsoever. I tried to put as much force as I could on it while pedalling.

This is probably the first time in many years (I would estimate 5 years) this bike has seen a chain replacement.

Anyhow, Generic; More frequent chain replacement helps to not have to buy a New Cassette .

If it skips just replacing the chain you waited too Long.

Remember to actually stretch your chain taut if you're doing pin to pin measurements.