This has been an intersting discussion, but as the thread starter I am still looking for some simple, practical guidance on the initial question: how to determine when to replace my KMC 11-speed chain. If the answer is that the Park chin measurement tool is adequate, then that's fine since I have one. If the answer is to use a steel rule to measure elongation, could I get some advice on which types or brands to buy, how long it should be and at what elongation point to replace the chain?
The Park chain tool is perfectly fine. The guideline from Campy when to replace a campy chain is when the distance, using good calipers is 132.60 between a certain number of links. I find this to be a pretty good guideline as to when I can start to notice a degradation in shifting performance. I've compared this measure to the Park guide and while the gauge doesn't quite fit in the 50% wear indicator at that point, it is close. I use the same measurement on any chain, be it Campy or Shimano or whatever. Some may call it conservative, but I'll get about two chains per cassette doing this. So, in the end where I used to use the caliper method, I'm equally fine with using the Park Tool and when I can fit the 50% wear indicator in there, it's time to replace. Letting it go to the 75% mark will just mean you spend a lot of time with sub par shifting and wear your cassette faster. Anybody who says that get 10,000K from a chain is, in my opinion, doing an awful lot of coasting.
I'm not even going to touch the religion of which lube to use and how to clean it. But I will say that I think the whole thing got blown out of proportion when the advocates of no degreasing were slammed. By not fully degreasing the chain, I don't think anyone was implying to just keep adding lube on top of grit and grime. It still needs to be cleaned. Soapy water in a Park Chain Cleaner will get the grit out, and is actually recommended by some Lube manufacturers so as not to remove the coating completely (Duo-Mond Tech comes to mind). Just make sure it is fully dry before you add fresh lube afterwards. Sitting in the sun will do this (I blow out mine with compressed air). Years ago Campy used to recommend using Kerosene for the simple reason that it did NOT completely strip out the lubricant and actually contained some lubricant properties itself. But they don't say that anymore, most likely for environmental reason. Kerosene is probably not be the most environmentally friendly of chain cleaners.
So use your Park chain measurement, replace when it is between 50% and 75%, and follow whatever chain cleaning and lube process is most convenient for you. You'll be good.