  For a relatively compact Stein domain $\Omega$ with $C^2$ boundary in a
Hermitian manifold $M$, we consider the strong Diederich-Forn{\ae}ss index,
denoted $DF(\Omega)$: the supremum of all exponents $0<\eta<1$ such that
eigenvalues of the complex Hessian of $-(-\rho)^\eta$ are bounded below by some
positive multiple of $(-\rho)^\eta$ on $\Omega$ for some $C^2$ defining
function $\rho$. We will show that $DF(\Omega)$ is completely characterized by
the existence of a Hermitian metric with curvature terms satisfying a certain
inequality when restricted to the null-space of the Levi-form.
