I did a bit more work on frontal area, which appears to support the short-cut we took earlier.
From a thread on Virtual Dyno on HpTuners, The quick way to calculate frontal area is (((w*h)(0.85))/12)/12 = Frontal Area.
For the 2007 base XLR, W=72.3in, H=50.4in so for a first approximation (((72.3*50.4)*.85)/12/12)= 21.5 ft2.
The question then is the 0.85 factor. To test that, I downloaded a diagram of the 2007 Cadillac XLR from the-blueprints.com and pulled just the frontal area into a photo editor.
Next, I selected the parts of the diagram that were NOT XLR
Then I used the photo editor (GIMP) to tell me how many pixels were shaded (20,668) of the total pixels size (149,464) which resolves to 20668/149464 = 13.8%, or 86.2% for the XLR.
That makes the frontal area for the 2007 base XLR, (((72.3*50.4)*.862)/12/12)= 21.8 ft2. In this case the approximation was good, but I will update my Virtual Dyno XLR profile to 21.8 ft2.