New to chain rotation: my second chain was a late delivery and as a consequence, the first one has racked up 3,750 km (~2,300 mi) like the rest of the drivetrain. I've kept it wiped and lubed regularly, lately almost on a daily basis to counter some rattling on the chainrings that has affected it in last 1,200 km.

Elongation only showed up recently, about 10 days ago, at 6.001 in. Today it's at 6.007 and the bike's weekly mileage is in the the 400 km range.

Since the chain is now getting longer at a measurable pace, what would the better course of action: just keep it running until it hits 0.5% or clean/relube/store/reuse?

You don't say how you're measuring it, but the methodology sounds suspect. In my experience, chains always show the fastest wear when new. The rate decrease as they get worn in. It's best and most accurate to use a ruler to determine chain wear. Here's why: https://pardo.net/bike/pic/fail-004/000.html

I replace chains at 1/32" elongation over 12". I don't rotate chains. But if I did, I only use chains with nearly the same mileage on them, perhaps <500 miles. Doing otherwise kinda defeats the purpose.

I don't think it's possible to measure chains over 6" accurately to thousandths of inches. If you're using a digital caliper, you're confusing resolution with precision

In any case, if the chain is well less than 1/2% worn, remove it, use the new chain now, and when the new one is more worn than this one switch back, playing leapfrog at shorter intervals going forward.

KMC has a Digital Chain checker.. for more accuracy on a short length, sampled.

@ Looigi, I'm using a 6in digital caliper with the chain mounted on the bike and under tension, measuring the upper run. Keeping the caliper's ends aligned against the pins just seems to come easier for me than using a ruler.

It does sound a bit too good to be true right? still, it lines up quite perfectly at 6.000in on the new chain, and on the old one too until a couple weeks ago. Over the past two months I tested it countless times and always got the same results, so it's consistent as well.

Cool, thank you. I was planning to get a third chain too, do you think that would be overkill?

FTR, the cassette is a SRAM PG-1070, and the cheapest price I can get a new one for, is ~2-2.5x that of a mid-to-high range KMC chain ...with sales those are usually less expensive than entry level models - go figure.

(BTW thanks for your many useful posts, I'm a bit of a lurker here and your contributions are always a very interesting read)

Same issues I discussed earlier. Better resolution or digital readings creates the illusion of accuracy and/or precision, but it doesn't necessarily follow. Precision and accuracy depend on methodology, and better resolution doesn't change that.

Measuring chains for stretch with a digital caliper is comparable to weighing yourself with a gram scale.

The third chain probably isn't overkill, but it depends on how much you ride, and your estimate of how long your cassette will last. I rotate 3 chains on my road bike and 4 on the commuter.