The tripods are designed to have all legs extended fully. Think about that for a moment........why? Because we want to measure when the torso passes the gate, not the legs. For example a lower leg hanging down when the femur is horizontal, could trigger the gate early. Extend to maximum length.
The only time you might want to have a gate set lower is when your athlete is in a crouch start position and might miss the top beam of the gate, or when the height of your athletes is lower such as young kids running some tests.