This threshold formulation helps explain why the Committee was willing to express the guidance bearing on the labor market in terms of the unemployment rate alone, instead of following its usual practice of considering a broad range of labor market indicators.
Estimates are highly uncertain, so it is best to put a range of +/- 50 basis points around these numbers. Global potential growth is not the only determinant of the neutral rate. There is also the increase in global savings witnessed over the past decade and a half, the lingering effects of the financial crisis on investment demand and post-crisis financial reforms.