  To further precision studies with $B \to K^{(*)} \ell \ell$ decays in the
high-$q^2$ window uncertainties related to the operator product expansion (OPE)
need to be scrutinized. How well can the OPE describe $B \to K^* (\to K \pi)
\ell \ell$ angular distributions for a given binning in view of the local charm
resonance structure? We present a data-driven method to access this
quantitatively. Our analysis suggests that the bins which are near the
kinematic endpoint are best described by the OPE and should be pursued for
precision studies. At the same time measurements with finer binning help
controlling the uncertainties.
