You could try using a WQL query - something like the following:
Multiply(Divide(ExecuteWql('Numeric', 'root\CIMV2', 'SELECT FreeSpace FROM Win32_LogicalDisk WHERE DeviceID ="D:"'), ExecuteWql('Numeric', 'root\CIMV2', 'SELECT Size FROM Win32_LogicalDisk WHERE DeviceID ="D:"')), 100)
This would be put in the Expression>Field cell (click the ... and directly paste the code). Then simply choose the >= operator and then give your percentage value that you want to alert on. Remember to change the drive letter to the drive you want to monitor.
I'm not sure what facet to use mind, I only played around for a few minutes to try and give you a solution 🙂 You might want to have a play around to see what gives the best results.
Hope this helps. 🙂