  In spin-orbit-coupled magnetic materials, the usually applied Curie-Weiss law
can break down. This is due to potentially sharp temperature-dependence of the
local magnetic moments. We therefore propose a modified Curie-Weiss formula
suitable for analysis of experimental susceptibility. We show for octahedrally
coordinated materials of $d^5$ filling that the Weiss constant obtained from
the improved formula is in excellent agreement with the calculated Weiss
constant from microscopic exchange interactions. Reanalyzing the measured
susceptibility of several Kitaev candidate materials with the modified formula
resolves apparent discrepancies between various experiments regarding the
magnitude and anisotropies of the underlying magnetic couplings.
