When measurements are not agreeing within 0.08% the problem might just be strict adherence to the "use the longer constant" rule, where you have perhaps calibrated in the early morning with cool conditions, and worked through the day as things warm up. I really think for experienced measurers that using the average constant will not only give you a closer approximation to the best possible measurement of the course, but will give you results that stand up well to a repeat measurement on a different day.