So, New Years Day I did a time trial. 3 mi 10:39. Last Friday I took the day off from work to TT again. But life the weather was not as kind there were +/- 20 mph winds. I decided I would do one going into the wind. Then one with the tailwind. Then take the two times, add them, then divide by two. Giving me a new time of 10.31. And I was wondering I can count this.
Unless you do all your riding in a lab, environmental variables will always factor into your TTs. I think your idea of two rides is OK. The one factor you can't account for is the fatigue on the second ride. It sounds like you are making progress and that's great!