My suggestion would be to follow the perimeter of the bumper fascia exactly like you see with current NASCAR .The bottom edge should be a little forward of the top and should be no lower than the lowest under-chassis components.
If you want to experiment with lower chin spoilers later,you can always add that.
The way you've illustrated your form looks like it will add so much additional frontal area that it will defeat some of the drag reduction.It's also so low that if you ever forget to climb a driveway ramp obliquely,that you'll rip your handiwork to shreds.
