I have found mine runs best in 5.75" of water. Thus, if my sump's water level is 10", I set the skimmer on a 4.25" shelf (acrylic or eggcrate stand). The skimmer is very sensitive to water level, so I constantly add top up water to maintain the sump water level the same. Dropping the water level more than 1/2 causes the skimmer to pretty much shut down, and raising it even slightly causes the skimmer to fill up with pretty clear water that isn't skimmed much, if at all.
The skimmer is sensitive to the water level at the output, which creates back pressure in conjunction with adjusting the gate valve.
I added a 5" piece of spa hose and a pvc elbow with a couple of holes drilled in it (both from HD) so the skimmer output is under the sump water level to quiet splashing.