I don't think they add your reaction time to your E.T. it's just the simple fact that a .500 light is exactly when the green light comes on(5 tenths of a sec. between each light coming down the tree)so the later that you leave obviously the longer it will take to get to the other end. The timer does not start until you have broken the starting beams, time only or bracket racing eather one it doesn't matter. A pro tree is 4 tenths of a second between all the lights coming on and the green coming on therefore a perfect light for the pros is a .400.