If you would like to see the factory belly pan set up on the 2010 Prius (.25 Cd) here are some pictures for your amusement. From a poster over at Prius Chat. Not 56K friendly
If I remember correctly there is a Mercedes Benz that has large sheet metal underbody, and it looked much smoother than this. It may have been a race car though. The one that gets 14mpg at 200mph, I think. Cool pics though.
...might be "...interesting..." project to simply "foam" the whole bottom and then 'hot-wire' shave the whole bottom flat, just to see just how "smooth" you need things.