Attic Efficiency: Check!

It was February 2009, and I had just finished the satisfying job of adding insulation.
After a quick look around the attic, I finally felt a sense of completion. I had finally covered the 3 major areas that my internet-based education had told me were the major causes of inefficient attics: lack of ventilation, lack of a barrier to radiated heat, and insufficient traditional insulation. It took several seasons of projects, a little bit of money, and the help of some neighbors, but I had finally managed to get my attic into some semblance of energy efficiency.
I am now tempted to attempt to perform a better analysis of how well this has worked for me. Since the only numbers I have used so far have been my electricity usage, I have been unable to account for some known large differences in the weather over the course of my improvements. Without this accounting, the numbers look quite good, as you can see from my previous posts on the subject.
But I have been doing some research into how one accounts for the known temperature differences from one year to the next in ascertaining the performance of one's home. For example, was my energy usage drop from 2006 to 2007 entirely because of the well-known fact that in my area, 2007 was a cloudy and cool summer? Is the fact that my 2008 usage is less than my 2007 usage even more impressive since 2008 was so much hotter?
It turns out that there are some ways to account for this. The concept of "degree-days" is used in conjunction with either heating or cooling. The base for "degree days" seems to be 65 degrees Fahrenheit. The concept is deceptively simple: for cooling, you look at how much hotter the daytime average was than the base of 65 degrees, and you add that number to your total. You continue adding (T - 65) for each of the days you are interested in, until you get a total the represents how much hotter it was outside than inside over the span of time you're studying.
There are complexities about how to best measure the degrees on a given day (the average is not all that accurate; you'd ideally want a measurement every hour or more), and inaccuracies in that heat flow is not always linear (i.e. a 20 degree difference from outside to inside might well be expected to use more than 2x the energy of a 10 degree difference) and there is a baseload entirely unrelated to heating or cooling. Nevertheless, using this method should be an improvement over using no method at all.
As it turns out, there are sources of data out there for Heating Degree Days and Cooling Degree Days for a lot of places out there. There can be holes in the data, which require filling from other sources that may not agree exactly, etc. In addition, there are many schools of thought on how best to utilize the data. When looking at energy usage, one it looking at a sum of many variables, so when looking at energy usage vs. Cooling Degree Days, one will still have other factors unrelated to cooling influence the outcome.
Nonetheless, performing some of this analysis might be instructive. Gird up your brains, faithful readers, for the upcoming analysis is not for the faint of math.