In Storage Manager, the Free (MB) displays the free space left *not counting autoextend*. For example, say I have a tablespace with one data file that is currently 2GB, but can grow to 4GB through autoextend. If nearly 2GB are used, TOra will display a free space of nearly 0MB. I believe it should indicate a free space of 2GB (50%).
It's suprising since the "Autoextend (MB)" column displays the correct maximum size :-)