A good wing official knows before the snap which yard line the runner needs to reach for the LTG. Other than maybe being directly on it, you treat it like the goal line. You are determining if the runner reached it or didn't. If they didn't, mark it 1/2 yard short so if you do a measurement an inaccurate clip/chain doesn't make it a first down. If you feel they did reach the line, mark it on or beyond the line for the same reason. One thing I wondered is if he knew the LTG, felt the runner reached it so killed the clock because he had a first down. When he came back he thought he was a half yard beyond the LTG, but when he peaked back he realized he had gone back too far. If that's the case, the mechanic backfired on him.