The Guardian article commits the mortal sin of not naming the study or its year or coauthors, so I can't be sure about this, but when I search Google Scholar for Paul Babiak, I find this 2013 paper by Babiak et al. (Search its title for the full text; I can't get the link to behave.)

It seems primarily to be about methodology, and gives means and correlations on its own scale but doesn't venture a conversion to more conventional measures; but when you get right down to what it's doing, it's based on anonymous assessments of respondents' bosses collected through Amazon's Mechanical Turk, each consisting of 20 questions on a 5-point scale. If this is the method that the study behind the Guardian article is using, I'm very skeptical of its diagnostic validity. Among other things.