  Aims: By utilising spectra of early-type stellar probes of known distances in
the same region of the sky, the large and small-scale (pc) structure of the
Galactic ISM can be investigated. This paper determines the variation in line
strength of CaII at 3933.661 A, as a function of probe separation for a sample
of stars, including many sightlines in the Magellanic Clouds. Methods:
FLAMES-GIRAFFE data taken with the VLT towards early-type stars in 3 Galactic &
4 Magellanic open clusters in CaII are used to obtain the velocity, EW, column
density and line width of IS Galactic Ca for a total of 657 stars, of which 443
are Magellanic sightlines. In each cluster there are 43-110 stars observed.
Additionally, FEROS and UVES CaII & NaI spectra of 21 Galactic & 154 Magellanic
early-type stars are presented and combined with data from the literature to
study the Ca column density/parallax relationship. Results: For the four
Magellanic clusters studied with FLAMES, the strength of the Galactic IS CaII K
EW over transverse scales from 0.05-9 pc is found to vary by factors of
1.8-3.0, corresponding to column density variations of 0.3-0.5 dex in the
optically-thin approximation. Using FLAMES, FEROS and UVES archive spectra, the
min and max reduced EW for MW gas is found to lie in the range 35-125 mA &
30-160 mA for CaII K and NaI D, respectively. The range is consistent with a
simple model of the ISM published by van Loon et al. (2009) consisting of
spherical cloudlets of filling factor 0.3, although other geometries are not
ruled out. Finally, the derived functional form for parallax and CaII column
density is found to be pi(mas)=1/(2.39e-13 x N(CaII)(cm-2)+0.11). Our derived
parallax is 25 per cent lower than predicted by Megier et al. (2009) at a
distance of 100 pc and 15% lower at a distance of 200 pc, reflecting
inhomogeneity in the CaII distribution in the different sightlines studied.
