I did the same thing with vanishing point and was as accurate as it was possible to get with the grid.
I don't like to admit it, but I think they were probably right, and he was onside. If you notice the half way line, there is a very slight "bend" in the middle of the pitch. If you bend the pink line slightly, so the centre of the line was touching Faes boot and both the edges where it touches the sideline were closer to our goal, he's probably just onside.