  Approximations of chiral SU(3) amplitudes at NNLO are proposed to facilitate
the extrapolation of lattice data to the physical meson masses. Inclusion of
NNLO terms is essential for investigating convergence properties of chiral
SU(3) and for determining low-energy constants in a controllable fashion. The
approximations are tested with recent lattice data for the ratio of decay
constants F_K/F_pi.
