You shouldn't need a valve on the air intake. The bubble (foam) height is the level that the bubbles inside the skimmer rise to. If you want to skim wet adjust your output so the collection cup fills every couple of days with light colored watery skimmate. If you want to skim dry adjust the output so that the collection cup fills with very dark thick skimmate after several days. If you get a lot of buildup on the inside of the riser tube it means you could stand to have to foam height higher.
The episode 88 series is all about euro reef skimmers and how they work
http://www.lafishguys.com/