First point to note is that Combined contribution limit works when we want to restrict EE contribution across multiple plans.
Combined limit does not work as a combination for EE and ER contribution. Example : If you want to restrict EE contribution for $25000 for Benefit plans 401K and 401T. In this scenario we can use the Combined limit functionality.
Your requirement seems to be different where you want to start the POST Tax contribution after the EE + ER reaches a particular limit. One solution which we have implemented is -
We have created 2 different plans: One for PRE TAX and one for POST TAX.
*First EE should be enrolled to PRE TAX plan only.
* We need to write a custom program which runs after each pay period is run to identify of his/her EE+ER contribution has reached the limit. This report will basically lists all the EE's who have reached their limit.
* For thes EE's you need to enroll them into PIST TAX plan before the next payroll run.
Thanks,