In previously reported calculations of the critical inclusion radius for reverse `spike' domain formation (Carey and Isaac 1964) it was necessary to estimate the magnetostatic energy of the inclusion after the reverse spike had formed. The necessity for this estimate is removed by recent calculations by Carey of the magnetostatic energies of spherical particles with various `free pole' arrangements. Accordingly, the critical inclusion radii for both cobalt and magnetoplumbite have been recalculated, again as a function of magnetic field. It is shown that introduction of the correct expression for the inclusion energy after spike formation leads to the critical inclusion radius being smaller and much less field dependent in both materials.