When we were pro tree racing, we would be at a different place every weekend. I would roll some numbers in so it would be real green for time runs/TnT and leave it alone. We were looking for consistency, difference in the lanes, lighting, sun and shade, etc. For first round we would roll numbers back out to try to get the light we wanted, based on that information.

If you go to the same place every week, it should be easy to get a baseline number.