Thank you for the replies. I completely agree that with a standard ruler the measurement should be done on a long section under tension.
I am doing the measurement while the chain is on the bike under tension and I hope to get a similar precision with a digital caliper on 5". The digital caliper I use seem to be accurate enough to give me the stretch at 5" (it has 0.0005" accuracy) as long as I measure it right.
So you guys also agree that these drop in chain checker tools are actually shorter than they should be. Have you ever measured one with a digital caliper?
What was the largest difference you have observed? (That is, when the tool drops in %0.5 what was the shortest real stretch you have measured?).
In my measurements chain checker seems to hit %0.5 when my measured wear is as small as %0.36-%0.38.
Also some chain checkers claim that new chain usually shows at least %25 but in my measurements it measured as little as %0.1.