I have an ATB cone skimmer and I can adjust the flow into the overflow cup by rotating the vertical pipe. This rotation opens or closes the output opening, with completly open causing the least overflow. The height of the water that the skimmer sits in has a large effect and it took me awhile to get that settled.
Every time I clean my skimmer it overflows like crazy, so I have to open the output to the max and run the skimmer with the drain open for a few hours until a coating builds up on the neck. Then I close the drain valve and cut the output down some.