I have only had poor results from 'chain checkers' - esp. 0.75% wear reading on chains new from the box. I don't know if it is a design issue or if the manufacturing tolerances on the tool are crap.
THe more reliable and simpler method is a ruler to measure 12 links (edge of pin to edge of pin) against 12 inches.
Although I generally do prophylactic chain replacements after a few 000s of kms (maybe at the start of the season or some other arbitrary point in time) without worrying about measuring anything, and then a 'functional' test for cassette wear - if the chain skips under load or otherwise doesn't mesh well with the cassette then the cassette is worn and needs replacement