UNDO is going to increase and decrease. It's utilisation can reach upto 100%. No worries. As per my knowledge we will increase the undo space only when oracle throw an error in alert log saying some SQL is failed due to insufficient space in UNDO/ROLL back segment.
In such case:
1. decrease the undo retention period
2. increase the undo space.
ORACLE will automatically release the space free space for 24hours.