I recived this email from Oehler regarding my chrono questions: So the good news is I can buy anything I need from Oehler and they seem to be very helpful. The bad news is I don't understand these instructions I interpret this to mean that I have an option of how far apart to put the screens, but that can't be right can it? Isn't the exact distance between the screens what measures the speed? What am I missing? Wanna kill these ads? We can help!