iPod touch Very complex sync with the iPod

Basically, rather than make a series of hand-made playlists, I'd like to sync about 5 GB of every song from a list of artists (using smart playlists), and then fill the rest of my 8 GB iPod with random selections from the rest of my library.

The problem with my current setup is that I'm using a smart playlist (only parameters are to exclude my artist-specific smart playlists) which I manually limit to however much free space is on my iPod Touch. Is there any way for me to make this "smart" playlist automatically determine how much data it should contain?

Thanks,

a.s