Personally I base my splitting on lintian's warning.
The package has a significant amount of architecture-independent data (over 4MB, or over 2MB and more than 50% of the package) in /usr/share but is an architecture-dependent package. This is wasteful of mirror space and bandwidth since it means distributing multiple copies of this data, one for each architecture.
<http://lintian.debian.org/tags/arch-dep-package-has-big-usr-share.html>