If you haven't been checking the chain some other way regularly, then do you really have a reason to doubt the tool? I've found the Park checker to not be terribly accurate, but it works for my purposes, which is to quickly check whether a chain is worn out. I don't care how worn out it is, it's just a yes or no question. If the chain isn't obviously fine on the Park tool, then I can assume it is worn out. The actual error in the tool is so small as to be insignificant unless you are trying to put a precise number on the wear.