Two-point boundary distortion estimate for Schwarzian derivative of holomorphic function

Let $f$ be a holomorphic function in the disk $|z|<1$, $|f(z)|<1$, and let $z_{1}, z_{2}$ are distinct boundary points of this disk in which the angular limits $f(z_{k})$, $k=1,2$, exist, $f(z_{1})\neq f(z_{2})$, $|f(z_{1})|=|f(z_{2})|=1$. Under some geometric constraints on $f$ the precise upper bound for $\textrm{Re} \{S_{f}(z_{1})+S_{f}(z_{2})\}$ is established. Here $S_{f}(z)$ means the Schwarzian derivative of the function $f$ at the point $z$.