  Unphysical effects associated with finite lattice spacing and partial
quenching generally lead to to the presence of unphysical terms in chiral
extrapolation formulae, which must be removed to make physical predictions. We
use mixed action effective field theory to discuss that through next-to-leading
order, simulations of meson systems which employ valence quarks satisfying the
Ginsparg-Wilson relation (lattice-chiral symmetry), depend only upon the
physical counterterms of interest, provided one uses a lattice-physical
(on-shell) renormalization scheme. With this lattice-physical renormalization,
the I = 2 pi-pi, I = 3/2 K-pi, I=1 K-K meson scattering lengths combined with
f_K / f_pi share only two linearly independent counterterms, both of which are
physical. This allows us to make use of existing lattice QCD simulations to
make a prediction of the I = 1 K-K scattering length.
