a&b,
Do you have an operating Weir door? If so, then we suggest the middle of the skimmer mouth.
The higher the water the less skimming action you get, and the lower the water the better skimming action you get.
If you have an auto-fill system, then I would run on the lower end... We recommend the middle, because it is the best compromise between skimming and constantly worrying about the water dropping below the skimmer mouth.
If you are having a skimming issue, and don't have a weir door, then I would get one installed.
Thanks,
Jim R.