Lastly, create a SQL Alert to notify you as soon as tempdb grows past the threshold you stipulate. Using the GUI to create the alert, you need to fill out every field on the General page and make sure the Enabled checkbox is marked. Create a Name for the alerts, then specify the Type as SQL Server performance condition alert. The Object should be Databases, the Counter is Data File(s) Size (KB), and the Instance will be tempdb. The alert will trigger if counter rises above the value. The Value will depend upon the cumulative size of your tempdb files. In this case each tempdb file is 12GB (or 12,288,000 KB), so the total size is 98,304,000 KB.
I liked the approach of only firing the SQL Agent job after a trigger was met, rather than running a job which queries and then creates an e-mail afterward.