Bill,
Every system is different. Each system has different bioload and different filtrating requirements. The water depth of a skimmer is a starting point. if your skimmer is working correctly, then there is no need to move it up or down.
We usually recommend 7"-8" on that skimmer but again, it is just a starting point. If you lower the skimmer, it may work or it may start overflowing on you.
If your skimmer is working, then don't move it.