... get total allocation for target segment - sum(bytes) from dba_extents
... calculate optimal allocation for target segment - assuming object was recently analyzed... num_rows * avg_row_len from dba_tables
... compare both - usually this does the trick with no further analysis.
... for fine grain results factor in pct_free/pct_free
Disclaimer: Advice is provided to the best of my knowledge but no implicit or explicit warranties are provided. Since the advisor explicitly encourages testing any and all suggestions on a test non-production environment advisor should not held liable or responsible for any actions taken based on the given advice.