I have heard of people fishing in 20 feet of water and having the bait only 5 feet under the ice. The theory: A pike can see up better than down because their eyes are on the top of their head. Thus making the bait in this situation easier to see for pike at all levels.
My theory: Start near the bottom and work your way up to where you start getting bites. If you have two tip-ups going, try different levels.