I just bought a Cateye CC-MC 100W. I had never been so confused with any operation manual no matter how sophiticated the gagdet is. But this Cateye manual had my head spinning. Just a couple of questions: 1) It says it can operate either in AT (Auto mode) or manual mode. What is the difference, particularly in terms of accuracies? What mode do you guys use? 2) It appears I don't need to calibrate anything if on AT mode. How then the computer or sensor knows the circumference of the wheel. 3) It doesn't switch off in AT mode. Is that correct? How to make it switch off.