SQL Server can automatically expand a database or log when necessary based on a set of parameters. The Auto-grow Settings tab of the Advanced Alert Configuration window allows you to specify whether you want to take this autogrowth into account when calculating the remaining space available.
SQL Diagnostic Manager allows you to select whether to trigger an alert based on the current database or log used size divided by the maximum possible size. Click Yes, alert on the current used size divided by the maximum possible size on the Autogrow Settings page to set the alert to include auto grow or click No, alert on the current used size divided by the current file size if you do not want to take SQL Server auto-growth into consideration.
Understanding the downside of Auto-grow
You can avoid unnecessary long reads and poor response time from SQL Server by minimizing the number of auto-grow events occurring in your environment. Each time an auto-growth event occurs, SQL Server holds up database processing while looking for disk space for the additional data. Usually this new space is not adjacent to the existing space and causes physical disk fragmentation. As more and more auto-growth events occur, the more the disk becomes fragmented, leading to longer periods while SQL Server reads the databases.