I asked this same question when I got my ASM......
First, it will take a while to "break in", but they say to start off you should raise the tube almost as high as it will go.....untill you start getting some skimmate (it will be very light in color and watery) And then, over time, you can lower it until the skimmate is to your liking (most people like the thicker, darker stuff).
Personally, I like it medium. My water quality is pretty good, so I don't get a ton of skimmate. If I lower it too much, I get nothing.