Below are some links that I thought you might find helpful.
I think this might be close to what you want. I used it to monitor a 10% increase or decrease. Not my code, I snagged it off here a few weeks ago. Create a custom SQL alert under the trigger condition.
LEFT JOIN VolumeUsage_Detail V
ON Volumes.VolumeID = V.VolumeID
AND Volumes.VolumeType = 'fixed disk'
AND DATEDIFF(HH,DateTime, GETDATE()) < 2
GROUP BY Volumes.NodeID, Volumes.VolumeID, Volumes.FullName
HAVING MAX(PercentDiskUsed) - MIN(PercentDiskUsed) > 10