Find optimal undo retention size - oracle database script

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 gv$datafile a,
     gv$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,  gv$parameter e,  gv$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';