What are your inactivity and absolute values?
If you set your inactivity value to 5 minutes, you should get pretty close to your goal.
If you set the inactivity value to 0, they would need to reauthenticate for every single http connection, IIRC = effectively unusable, so you need to live with somevalue > 1, which conceivably could mean that a user could close the browser completely, and reopen it, and not need to reauth