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I think it would be best to stick with a simple coast down timing aid rather then pretending to calculate an accurate CD. A simple timing aid would improve coast down timing over a stop watch and could be every bit as useful for comparing aero mods as an estimate of CD.
Unless the instrument logs wether data you need to keep track of that separately. Enter the weather data and coast down info in a spread sheet and do the data reduction where you can work comfortably.
To compare data from different days requires correcting for air density. Of course you would always use the same road with multiple runs in each direction and average. If your looking at fine tuning you get excited about drag changes of 1%. Trying to measure that amongst the affect of things like light breezes, changes in air density, and lubricant viscosity changes with temperature requires carefully correcting for known variations to pull something very small out of a whole lot of noise and variations.
With an aero mod that is easy to install/remove you can do ABA testing in one session so you would not need to make any corrections to timing data to evaluate a drag change.
Mike
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