The new version will also be slightly longer than the first. I've checked out the rear view mirror, and on a flat road, can still (just) see the horizon, below the lip of the extension.
The main reason I want to tuft test it is because even though I'm following the established curve, the "roof line" angle I measure through the end of the cardboard (though it's slightly curved, not flat) is 14.5 degrees. That's about the same as the rear roofline of the Audi A2 (Cd 0.25), but several degrees more than the Insight (Cd 0.25) or Prius (Cd 0.26) - about 12 degrees each. (Though without its rear spoiler, the Prius is about 14.)
I want to make sure there's still attached flow at the end before putting in the hours to do the final version.