  We present a simple model for the host mass dependence of the galaxy
nucleation fraction ($f_{nuc}$), the galaxy's nuclear star cluster (NSC) mass
and the mass in its surviving globular clusters ($M_{GC,obs}$). Considering the
mass and orbital evolution of a GC in a galaxy potential, we define a critical
mass limit ($M_{GC,lim}$) above which a GC can simultaneously in-spiral to the
galaxy centre due to dynamical friction and survive tidal dissolution, to build
up the NSC. The analytic expression for this threshold mass allows us to model
the nucleation fraction for populations of galaxies. We find that the slope and
curvature of the initial galaxy size-mass relation is the most important factor
(with the shape of the GC mass function a secondary effect) setting the
fraction of galaxies that are nucleated at a given mass. The well defined
skew-normal $f_{nuc} - M_{gal}$ observations in galaxy cluster populations are
naturally reproduced in these models, provided there is an inflection in the
{initial} size-mass relation at $M_{gal} \sim 10^{9.5} {\rm M_{\odot}}$. Our
analytic model also predicts limits to the $M_{gal} - M_{GC,tot}$ and $M_{gal}
- M_{NSC}$ relations which bound the scatter of the observational data.
Moreoever, we illustrate how these scaling relations and $f_{nuc}$ vary if the
star cluster formation efficiency, GC mass function, galaxy environment or
galaxy size-mass relation are altered. Two key predictions of our model are: 1)
galaxies with NSC masses greater than their GC system masses are more compact
at fixed stellar mass, and 2) the fraction of nucleated galaxies at fixed
galaxy mass is higher in denser environments. That a single model framework can
reproduce both the NSC and GC scaling relations provides strong evidence that
GC in-spiral is an important mechanism for NSC formation.
