  When thermal rate equations are derived for the evolution of slow variables,
it is often practical to parametrize the right-hand side with chemical
potentials. To close the system, the chemical potentials are subsequently
re-expressed in terms of the slow variables, which involves the consideration
of a "susceptibility". Here we study a non-relativistic situation in which
chemical potentials are large compared with the temperature, as is relevant for
late-time pair annihilations in dark matter freeze-out. An order-of-magnitude
estimate and a lattice simulation are presented for a susceptibility dominated
by bound states of stop-like mediators. After this "calibration", the formalism
is applied to a model with Majorana singlet dark matter, confirming that masses
up to the multi-TeV domain are viable in the presence of sufficient (though not
beyond a limit) mass degeneracy in the dark sector.
