There's probably a million threads on this very question, but I couldn't figure out how to search for it.
Short version: Is a chain wear tool a valid screening tool - does it err on the side of falsely indicating excessive wear (good for screening) or the opposite - tricking you into thinking the chain is OK when really it's worn (bad for screening)?
Long version: I totally understand that it is cheap and easy to measure chain wear using a steel ruler and the 1/16" elongation criterion. I can do that. But I also understand that it's best if the chain is under at least a little tension (i.e. hanging from a nail or manually stretched out.
I also understand that using a tool is dead simple but might not accurately measure real chain wear (I can't explain to you why but when i read explanations, it makes sense, so there).
Finally, I also think it's a hassle with poor lighting and only two hands to do a truly accurate measurement with a ruler while a chain is on the bike - pulling the chain in tension, holding the ruler precisely and sighting a 1/16 measurement. Not impossible mind you, but more of a hassle than just using the chain wear indicator tool ... again, while the chain is on the bike.
In my family of fairly active riders, I have about a dozen chains I need to keep track of, mostly on bikes but also a couple of spares I rotate in. I have a Park chain wear tool and a steel ruler. Yesterday I did a spot check of a couple of chains on the bikes with the Park tool and also the spare using the tool and a ruler (which has similar wear).
I did 2-3 checks on each of the chains I checked with the tool on the bikes. A couple of chains were totally "OK" with the tool at every test site. A couple were OK at two sites, but showed ".75%" wear on a third. In other words, those chains could have shown "OK" if I'd only checked once.
This is where the question comes in. If the indicator tool shows wear, I'm thinking then I'd take the chain off and do a ruler test to confirm. But, if the indicator tool doesn't show at least "75%" wear, I'll just lube up the chain and we're good to go.
What do you think?