I set mine fairly low. It still skims but the skimmate I get is very think and gunky. When I set it higher I get quite a bit of liquid, but not very gunky. I prefer to empty thick gook every 2 weeks or so rather than thin green water every couple days.
Actually I believe optimal is probably somewhere right in the middle. Take some time and do very small adjustments one way or the other until your happy with it. By "some time" I mean several weeks.
It also helps to keep your water level of your sump consistent which is something I'm not really good at.
I'm very happy with my skimmer. Is yours a true Euroreef or an ASM like mine?