Here's why I disgaree. Foam, whether it is in the neck or skimmate cup, is not in the water column. That is the goal. The biochemical nature of skimmers wont allow the protein enriched foam back into the water column. Most skimmers I know of are set up by water level, not by foam level. The foam level will vary, but water level is controlled by input and output. The longer the water is in contact with the bubbles, the cleaner it will become.
Heres a simplistic way to look at it. A skimmer is to your tank, as soap is to your skin. You can't rub a bar of dry soap on your skin, and expect it to clean it. You need to make lather (ie- foam in the skimmer). These little bubbles attract the polar chain organics have molecularly. That is what takes iof the "dirt". Now, if you scrub you hands for 1 second and rinse, will the be as clean as if you scrub for 10, 20, 30, etc seconds?
The skimmate cup really does NEED to be filled. It usually does get filled as these organics are removed from the water, but if the skimmer is really tall, and the foam ias building and buidling inside the neck, you may not see skimmate in the cup. Is the skimmer working- absolutely.
A large skimmer will likely work much much better on a small tank, with no more adjustment than a smaller. If your goal is to get the skimmer to make something for the cup, then yes, you will needs to adjust the valves. But, your skimmer will be working, even if you dont see much skimmate.