You should lower it to a few inches below the tile. Ice can pop the tiles off here and there. As far as the 8 inhes goes, that depends on if you have a solid or mesh cover. Normally if you have a mesh cover, you will need to pump some water out of the pool at some point to keep water below the tiles before the top freezes over solid. As far as lowering past the returns, you can do that, but it's really not needed. You can blow the lines without issue with the water above the returns and skimmer.