Basjoos, thank you for your detailed reply. Perhaps I used the wrong terminology, as I'm planning to have just a slant back nose, and nothing underneath it, i.e. the plastic nose will serve as a combination grill / nose / and semi airdam depending on the height in the front. I'm not planning to have anything underneath it except space, as that's how The donkey CRX had his for awhile, but might add a bottom later on. The car has a good belly already, so my question is really about the height of the wrap around slant back nose, similar to the front black piece that The donkey CRX uses and the aluminum on the front of Aerohead's truck.
Based on your comment of the solar racing cars, perhaps 8 inches will be fine with the slant back, then focusing on streamlining the smoothness underneath. However, the solar racing cars travel at relatively low speeds, and I wonder if their belly height would be as beneficial up to 70-75 mph, plus I think their bellies are closed off completely. Of course, if I cut the material to 8 inches, then I won't be able to lower it from that point.
With the curved around slope, the height could be 6 to 8 inches in the middle, and 4 to 6 inches in front of the tires.
The side trim on my car is made of metal, not rubber. That is interesting that you filled in the indents.