Update: I switched from Park Tool chain checker to measuring with a ruler and I'm not going back
Ran a worn chain on a customer's commuter last week using the tool, it read 0.5. Pulled out a 12 inch steel rule and measured 12 full links pin to pin, got 12 1/16. The chain was actually past 0.75. Tool drops into the roller gap and roller slop gives you a false reading. The ruler doesn't lie. Anyone else measuring by hand or am I the odd one out here?