a little bit lower than half way. Make sure its not to low or the pressure on the tiles will decress so the tiles might start to become undone and if you have marble dust it will give out. Make sure its not to high where if it froze it would over flow or even crack.
You need to drain the pool so that the water level is at least 6 inches below your filter and skimmer intakes. This will prevent water from freezing in them.