As long as you're within the water line, you should be fine. I think it's between 8-10" if it's an in sump skimmer.
What affects the consistency of the skimmate is not the water line, but how dialed in the valve is on the skimmer. A dryer skimmer with the gate valve open, a wetter skimmer with the gate valve closed etc.
As for optimal skimming, as long as you top off your water to keep the waterline where it should be, and dial in your skimmer to give you a decent skim you'll be fine. It takes time to find the right setting (more wet/dry etc). Leave the skimmer alone overnight and see how you like the skimmate produced. Adjust as needed and test overnight again and so on until you find the consistency you like. I like mine on the dryer side with only a bit of water collected. Not sure how it's "supposed" to be but i like the results i'm getting.