The water level externally has almost no impact on the internal level. What does have an impact is how fast you are feeding water through the skimmer. If you try and feed more, the water rises. Different pumps, pump ages, cleanliness of pump will all have an impact on internal water level although once you have a consistent setup, the skimmer will operate pretty consistently as well. If your skimmer is plumbed via a manifold attachment or gravity feed startup is going to have an impact on the internal water level. Added nutrients to the system is also going to have an impact on foam level, some foods are worse than others. By lowering the standpipe section below the gate valve you lower the the lowest point the skimmer can be set at internally. This means that you can run more water at a faster feed rate through the skimmer and not overflow. Most people feed the skimmer too much water, I have posted Escobal's turnover calculations for skimmers many times here so a search should pull up the formula if your interested in what is "optimal".