Yeah, what you'd have to do is know all of the early ones, then think, "Okay, I'm so far up now that the audience probably isn't going to do me much good anymore; I might as well use them now, and if the vote is inconclusive, hem and haw a while, then use the PaF."
That said, yes it can be a good strategic move, but in all likelihood, you aren't going to see it used, except perhaps by accident, i.e., bad contestant uses two lifelines on question, PaF happens to be on MillionaireIM list.