The purpose of the parameter "Pool entrance depth" is to describe the distance a fish would need to leap in order to enter the culvert for the AOP score. I suggest measuring from directly below the outlet for simplicity sake and safety (you cannot enter the culvert). I would recommend doing the same for measuring hydraulic control distance- just use the outlet as you endpoint. Add in the comments that the pool actually extends into the culvert itself and use your range finder or a visual estimate to include that additional distance. Is the outlet backwatered then? Be sure to select that option for "outlet grade".