[Sven7]

Flow Illustrator isn't a testing program so don't expect "results" from it.

The point of an air dam is to direct air away from the "dirty" underbody. The point of a belly pan is to make the underbody "clean". If you want to kill two birds with one stone just keep the pan at precisely a zero degree (horizontal) angle to the ground. This will prevent it from compressing air under the car, and you should reap the benefits of both aero aids.

For a fastback profile you'll want the rear end (behind the rear axle or wherever is convenient) to angle upward at a 3 degree angle from horizontal for maximum drag reduction. Anything steeper than about 7* back there will actually increase your drag.