  We implement a proposal made in [arXiv:1107.4388] to determine the lattice
spacing by matching the lattice vector correlator at a reference distance scale
with the same correlator obtained by a dispersion relation based on the
$R$-ratio determined experimentally. We work with the isovector current,
requiring a separation of the isovector hadronic final states on the
phenomenological side. We also discuss the finite-size effect on the
correlator, which must be controlled in order for the method to be applicable.
