Using flashback queries.

 Hi

I have a  fixed tablespace of size 32 gigs with Undo retention set to 1800 secs. The retention gurantee isnt set. If I want to use a flashback query as of timestamp (sysdate-3), do I need to increase the undo retention and apply retention gurantee? There are some over heads of using retention gurantee, like the dml might fail since the undo tbs isnt autoextending. What is the best way to acheive the flashback upto maximum 5 days. I cannot change the undo tbs to autoextend du to reasons.

Thanks in advance for the answer.

Regards

KLA