Values for predicted value of average magnetic field calculated from Eqs. (2) and (3) valid for all masses. The dashed gray line indicates identity between ⟨B⟩ and B_pred, and the histogram shows the distribution of ⟨B⟩ around B_pred. We derive from the histogram that our relation estimates the value for ⟨B⟩ within roughly a factor of two uncertainty; this range is indicated with the two dotted lines.