I would avoid the use of OPTIMAL storage parameter as this will lead to dynamic allocation and deallocation of the extents which will lead to low performance.Instead of setting optimal storage parameter,set proper values for INITIAL & NEXT storage.
You can get to know how much of undo information is generated for you transaction by checking the
'USED_UBLK' column of v$transaction after executing the transaction.
This will give u the no of undo blocks used by the transaction.
Multiply this by ur block size and round it to the next multiple of power 2.
Set the result got as the value for the INITIAL and NEXT storage parameter and set ur minextents to 20.

HOPE THIS HELPS.

REGARDS
ANANDKL