The best explanation of chain wear measuring tools I know of is this:
http://pardo.net/bike/pic/fail-004/0...easuring-tools
The bottom line is that he finds commercial chain wear measuring tools are conservative in their wear estimates. But also that almost all of them includes roller wear in their measurements instead of just measuring pin wear.
A tool series that only measures pin wear is Shimano's TL-CN40/TL-CN41 and the new TL-CN42.