  It is common practice to truncate the finite-volume formula for $K\to\pi\pi$,
and other one-to-two transitions, to only include the lowest partial wave, as
in the original derivation by Lellouch and L\"uscher. However, as the precision
of lattice calculations increases, it may become important to asses the
systematic effect of this approximation. With this motivation, we compare the
$S$-wave-only ($\ell=0$) results with those truncated at the next lowest value
of angular momentum.
