Here’s what Google’s John Mueller said in response to this question on a Google Webmaster Hangout,
My general recommendation here is not to focus on these kinds of artificial metrics… because it’s something where on the one hand you can’t reproduce this metric directly because it’s based on the overall index of all of the content on the web.
So it’s not that you can kind of like say well, this is what I need to do, because you don’t really have that metric overall.
Perhaps most interesting is that he also added,
this is a fairly old metric and things have evolved quite a bit over the years
Hinting that Google has little to no use for this specific metric anymore.
That’s not to say it can’t be useful. It’s good to get a run down of your content and see terms and phrases others are focused on that you aren’t, but it probably isn’t the be-all and end all to what you need to do to rank.