Modeling Open Bottom Chamber

Hi everyone! I was hoping someone could help me model a very basic underground infiltration basin - I very rarely (if ever) use HydroCad and my coworker who typically handles this is out on bereavement so I really don't want to bother her. I need to model an open bottom chamber (such as a Contech ChamberMaxx) with the primary outlet being exfiltration. I can properly model the subcats/area nodes and rout them to the pond and use the chamber wizard to model the basin but where I get lost is when entering the parameters for the exfiltration as an outlet. The design is such that the chambers will sit on 12" of crushed stone and then in conjunction with exfiltration there will be a small (about 2" dia.) overflow pipe situated at the top of the basin should there be a storm greater than the design storm and the basin begins to fill up(this is a parameter set forth by the governing city). A perc test was done and the soil has a saturated perc rate of 30"/hr. with the SHWT greater than 15' below the bottom of the proposed basin. Any help in properly inputting the exfiltration would be much appreciated as I am torn between using "constant velocity" or "conductivity" and the specific fields that come with each.

Any help would be VERY much appreciated - Thank you!