  By applying different statistically robust methods, we analyze the time-lag
between the continuum and ionized line-emission (Mg II line) light curves for
the distant bright quasar CTS C$30.10$ (redshift $z \sim 0.9$). The data were
obtained by the 10-meter SALT telescope in South Africa. In detail, we
demonstrate the application of several methods using interpolated
cross-correlation function (ICCF), discrete correlation function (DCF),
z-transformed discrete correlation function (zDCF), von Neumann estimator, and
the JAVELIN code package. In particular, we discuss the uncertainties of these
methods. In conclusion, we find that the quasar lies on the broad-line region
(BLR) size -- monochromatic luminosity power-law scaling, $R_{\rm BLR}\propto
L_{5100}^{1/2}$, which was already confirmed for low-redshift sources. In case
the BLR size-luminosity relation holds for other distant sources, quasars could
be used for probing cosmological models as ``standard candles'' complementary
to supernovae Ia.
