Before Comey's interference, Clinton was ahead of Donald Trump nationally by an average of around 5 points. After Comey's interference, Hillary's nationwide polling average dropped to around a 2 point lead. Her polling average dropped by around 3 points nationwide after Comey.
But how much of that was Clinton dropping as opposed to Trump gaining at the expense of Johnson and Undecided? What's unclear to me is how much movement from Johnson and Undecided to Trump was going to happen as the election approached regardless of what campaign events were occurring.