  An indirect CP-violation neutrino factory experiment is studied, including
simul taneous correlations of errors of the six oscillation parameters and a
matter ef fect normalisation parameter, C. It is found that for larger values
of $\theta_{13}$ ($3^\circ$), an uncertainty on the matter effect of the order
5% means t hat the optimal baseline for measurement of $\delta$ is reduced from
3000km to 2 000km. When the background fraction is assumed to be $10^{-3}$ the
optimal muon energy is also reduced from 50GeV to around 20GeV. An experiment
with L=2000km, $E_\mu=$20GeV is found to be sensitive to non-zero $\delta$ down
to around $\t heta_{13}\approx0.3^\circ$.
