undo tablespace tuning

I am trying to tune my undo tablespace and see if I need to increase the size to make it optimal. I ran two different queried and got two different results. I am not sure which one to follow.

The first one I follow the formula UndoSpace = [UR * (UPS * DBS)] + (DBS * 24)

SELECT (UR * (UPS * DBS)) + (DBS * 24) AS "Bytes"
FROM (SELECT value AS UR FROM v$parameter WHERE name = 'undo_retention'),
(SELECT (SUM(undoblks)/SUM(((end_time - begin_time)*86400))) AS UPS FROM v$undostat),
(select block_size as DBS from dba_tablespaces where tablespace_name=
(select value from v$parameter where name = 'undo_tablespace'));