Push down with 1 finger in the middle of the chain. This will apply roughly the correct pressure. Then measure from bottom of swing arm to the top of the chain link. Its not an exacting science but will give you a good indication.
I noticed same with my 2021 XSR700. I had driven it about 500 km, and saw that the chain is really loose. I think the factory chains are not pre-stretched (perhaps different year models come with different chains), so it is normal for them to stretch after 100 km or so. I also got it tightened, and went to buy 27mm socket and a breaker bar.Stopped by at the dealer for second opinion, yes it was sloppy and streched. They fixed it for free in 15 minutes. Without even booking it in.
We all know this. But that is just the established term here in normal speak. Even chain manufacturers use that. It looks like many chains have the initial wear greater than what happens after it (as you explained). I think stretch is better term here than wear. Rusty chains have different kind of wear. In Finnish we never talk about stretch, we just say that chain is loose.The chain does not stretch