This is one of the most commonly asked questions. The answer is not straight forward but also not complicated.
There are many factors that can influence the efficiency of a protein skimmer. Salinity level, water tension, water chemistry to even how long the skimmer has been running for.
A good rule of thumb is to pay close attention to the size of the foam bubbles inside the skimmer chamber. You want pea-sized and large-size bubbles to slightly extend over the line where the bottom of the collection cup meets the skimmer's body.
If you have a new skimmer, this may need to be adjusted to a lower level until the skimmer's break-in period is over. The break-in period of a skimmer is usually 3 to 4 weeks. Once the skimmer's break-in period has past, adjusting the skimmer level is a much easier task.