Monitoring FileGroup FreeSpace to Avoid Autogrow

I have a large database 300GB that was recently divided into filegroups to better facilitate backup and recovery. The application that uses this database slows considerably if an autogrow occurs during normal business hours. I have been using sp_spaceused on each of the tables in the filegroup to determine free space and then schedule an off hours MODIFY FILE to add needed space. But if DBCC UPDATEUSAGE is not run consistently, I cannot depend on the sp_spaceused results. Is there a better way to determine when a file/filegroup is nearing capacity?

Thanks in advance.

Thank you for the information