It can take some time to break in. I like to set mine with the water line right in line with the lifereef sticker (about 1 1/2 to two fingers from the top). I close the drain and each day when I get home if no skimmate is in the cup I turn the gate valve about 1/16th turn. When you see it getting close make very small turns less than 1/32nd and you can dial it in to the perfect skim that your looking for.
I judge the production based on the entire days results instead of what you see while your tuning. Throughout the day the foam head can rise and fall based on barometric pressure and other things that could enter your water like fish food or oils from a hand. I find the daily average is the best way to judge how effective the skimmer is performing so always start on the cautious side as to not over flow the cup before you get home.
The good thing is if you got everything hooked up properly it will tune itself down when it draws water from the cup to the Venturi and not make a mess even if your running external.