I've wrestled with this issue on how to detect if a Mac OSX hard drive is getting close to full. All of our Macs have multiple disks and some are systems disks/partitions that are full by default. Here's how I did it.

1) First make a report call it whatever you want.

2) The topic will be disk and select the fields you want

3) Set the Filter to the follow options

Now you will be able to run a report and find machines that are close between 90% to 100% full and also have less then 10gb free and won't get those pesky system disks as a false positive.

Cheers,

M