[aardvarcus]

Going back to "A" helps to average out any slight changes in the weather pattern or road conditions. For example if it takes 30 minutes to do a route and 30 minutes to change your configuration, it is possible that the temperature has warmed up (morning) or cooled off (evening) in the hour between the tests. Assuming these changes are close enough to linear, the ABA will factor most of that out once you average A1 and A2.

Make your route long enough to be credible. Also, since you are testing aerodynamics with the belly pan, a higher speed test will show the differences more. Interstate or highway is best. Also, minimize things you have no control of. Try to avoid traffic, red lights, etcetera. Testing at an odd hour on a four lane would be best, minimal traffic to avoid and an extra lane to avoid them in. Also, use cruise control to the extent possible if you have it. Cruise should react basically the same on all three runs, to prevent you from accidentally accelerating hard up a hill on one run and easy on the next.

Lastly, as others have stated make sure your car is fully heated up before you start. Using a gauge, monitor your intake temperatures as well as your engine temperatures. Due to heat soak, my intake temperature continues to rise long after my engine temperature has flattened out.

Edit-
Oh, and one other item that would make your belly pan test really great. Get a non-contact thermometer and after each run get the temperature of the Engine Pan, Transmission Pan, Gas Tank, and Rear Differential.