75% is when I start cleaning up. My 120GB SSD isn't very big, so that happens fairly often during the Steam Sales, but otherwise, everything else goes onto my 1TB Data Drive, or 2TB Program/Games array.
Same for my laptop. Programs, documents, music go on the 120GB SSD, videos and other downloads go on the 640GB HDD (500GB Data Partition) and time machine backs up the SSD to a 140GB Backup Partition on that HDD.
I hate trying to work with limited space and having to manage different drives, but SSDs are worth it.