One thing with that rule: It'll alert if the EventInfo includes "80." If your disk makes it from 79.4% full to 80.5% full in one cycle (about an hour) that rule won't fire because "81" =/= "80"
There is a template rule for disk alerts on the LEM called "SolarWinds Disk Warnings" that was written to alert if any of the disk partitions on the appliance exceed 90%.