  Cosmological gravitational microlensing has been proven to be a powerful tool
to constrain the structure of multiply imaged quasars, especially the accretion
disc and central supermassive black-hole system. However, the derived
constraints on models may be affected by large systematic errors introduced in
the various stages of modelling, namely, the macromodels, the microlensing
magnification maps, and the convolution with realistic disc profiles. In
particular, it has been known that different macromodels of the galaxy lens
that fit the observations equally well, can lead to different values of
convergence, $\kappa$, and shear, $\gamma$, required to generate magnification
maps. So far, 25 microlensed quasars have been studied using microlensing
techniques, where each system has been modelled and analyzed individually, or
in small samples. This is about to change due to the upcoming synoptic all-sky
surveys, which are expected to discover thousands of quasars suitable for
microlensing studies. In the present study we investigate the connection
between macromodels of the galaxy lens and microlensing magnification maps
throughout the parameter space in preparation for future studies of large
statistical samples of systems displaying microlensing. In particular, we use
55,900 maps produced by the GERLUMPH parameter survey (available online at
http://gerlumph.swin.edu.au ) and identify regions of parameter space where
macromodel uncertainties ($\Delta\kappa$,$\Delta\gamma$) lead to statistically
different magnification maps. Strategies for mitigating the effect of
$\Delta\kappa$,$\Delta\gamma$ uncertainties are discussed in order to
understand and control this potential source of systematic errors in accretion
disc constraints derived from microlensing.
