dynamic NAT

Hi, I'm wondering if someone can give some suggestion for nat-int-increment and nat-test-increment using sip-dynamic-hnt.  Currently, we don't use it and hard set our nat-interval at 60 seconds.  Originally, this was provisioned on a new SBC with nat-int-increment set at 20 and nat-test-increment set at 15.  With this configuration, the nat-interval was also set down to 30, since the first register would send out an expires of 50 seconds.  The majority of the endpoints ended up with an expires time of 30 or 45 seconds.  This is worse than not using they dynamic nat and just making everyone use 60.