  We discuss the reduction of errors in the calculation of the form factor
$f_+^{K \pi}(0)$ with HISQ fermions on the $N_f=2+1+1$ MILC configurations from
increased statistics on some key ensembles, new data on ensembles with lattice
spacings down to 0.042 fm and the study of finite-volume effects within
staggered ChPT. We also study the implications for the unitarity of the CKM
matrix in the first row and for current tensions with leptonic determinations
of $\vert V_{us}\vert$.
