That is my "go to" sampling box and what I run most of the time. I really like the thing and it resides in the Jeep at all times.
I'd start off with about a 2" drop on it, and watch how your material flows. If it blows right on through, either slow down the water coming in or raise the bottom end a little. If the riffles are getting filled either increas the flow or add a little more drop.