Is it possible to build logic into an alert that will do the following?
Set the level of criticality:
if ( 10% < disk_free% < 15%) : alert level = Warning
if ( 5% < disk_free% < 10%) : alert level = Major
if ( 0% < disk_free% < 5% ) : alert level = Critical
--> this would utilize the trigger conditions, but as you can see, it is a nested condition that needs to affect another aspect of the alert.
Set the alert frequency:
if ( alert level = Warning ) : email frequency = every 24 hours
if ( alert level = Major ) : email frequency = every 8 hours
if ( alert level = Critical ) : email frequency = every hour
The result would be this: a single alert with reset condition that escalates both the alert level as well as the reminder frequency depending on the alert level.