I found a section of another fluids textbook online that has some characterization of different shapes (specifically ellipsoids of varying geometric length to height ratios) and offers some additional information about drag coefficients.
203.158.253.140/media/e-Book/Engineer/Fluid%20Mechanic/Fundamentals%20of%20Thermo-Fluid%20Sciences/cen54261_ch15.pdf
Hopefully this will help you out a bit more if you are stuck on your extrapolation argument. Additionally, I would recommend the Blevins applied fluids mechanics text, which I'm guessing can be found in the Dude.