Script to find optimal UNDO retention size in Oracle

SELECT d.undo_size / (1024 * 1024) "ACTUAL UNDO SIZE [MByte]",
       SUBSTR(e.value, 1, 25) "UNDO RETENTION [Sec]",
       (TO_NUMBER(e.value) * TO_NUMBER(f.value) * g.undo_block_per_sec) /
       (1024 * 1024) "NEEDED UNDO SIZE [MByte]"
  FROM (SELECT SUM(a.bytes) undo_size
          FROM v$datafile a, v$tablespace b, dba_tablespaces c
         WHERE c.contents = 'UNDO'
           AND c.status = 'ONLINE'
           AND b.name = c.tablespace_name
           AND a.ts# = b.ts#) d,
       v$parameter e,
       v$parameter f,
       (SELECT MAX(undoblks / ((end_time - begin_time) * 3600 * 24)) undo_block_per_sec
          FROM v$undostat) g
 WHERE e.name = 'undo_retention'
   AND f.name = 'db_block_size';