OEM Tablespace Columns Explanation For Help Deciding When to Add Datafile

Oracle database 12.2

OEM Cloud Control 13c

A particular tablespace has 1 datafile. The datafile is set to autoextend. 

However, the datafile has grown to 32GB. The tablespace is not a Bigfile tablespace, so if I'm not mistaken, the datafile cannot grow beyond 32GB.

Oracle Enterprise Manager reports that the "Space Used" is 31.344GB (see pic below). The tablespace grows pretty slowly, but it is growing. So, I am concerned about the tablespace running out of space.

However, I see that the "Available Space Used" column reports 63.35% used.

Also, when I query the dba_tablespace_usage_metrics, dba_data_files, and dba_free_space views, I get results that indicate roughly 20GB used and 12GB free in the tablespace (which when translated to a percentage agrees with the number reported by the "Available Space Used" column.)