We report on a new method to determine the degree of bulk spin polarization in single crystal Co$_{(1-x)}$Fe$_x$S$_2$ by modeling magnetic Compton scattering with {it ab initio} calculations. Spin-dependent Compton profiles were measured for CoS$_2$ and Co$_{0.9}$Fe$_{0.1}$S$_2$. The {it ab initio} calculations were then refined by rigidly shifting the bands to provide the best fit between the calculated and experimental directional profiles for each sample. The bulk spin polarizations, $P$, corresponding to the spin-polarized density of states at the Fermi level, were then extracted from the {it refined} calculations. The values were found to be $P=-72 pm 6 %$ and $P=18 pm 7%$ for CoS$_2$ and Co$_{0.9}$Fe$_{0.1}$S$_2$ respectively. Furthermore, determinations of $P$ weighted by the Fermi velocity ($v_F$ or $v_F^2$) were obtained, permitting a rigorous comparison with other experimental data and highlighting the experimental dependence of $P$ on $v_F$.
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