On my Primary drain stand pipe, you can see I have about 5-6 inches of holes for water to flow through. So any where along that distance, I will have a water level above the drain that will not allow air in. A silent drain
Don't get hung up on this distance or that. The only point about the water hight, is that it provides some self regulation. You do not have to adjust the gate valve to a gnat's *** to get it perfectly balanced.
With out taking into account of head pressure, you would have to set the gate valve absolutely perfect to match what is coming from the return pump, and what you are draining from the tank. But you don't have to.
If you close it a "little" too much, water level will rise giving more head pressure, allowing more flow, and matching flows. Take head pressure out of that and the tank will just continue to raise water level and over flow.
My only advice to you is where you set your emergency drain. You tank overflows to your box. Set you emergency drain right above your normal overflow height. Say your overflow is 2 inches below the top of your tank. You set your emergency drain just below the top of your tank to prevent overflow. Well now you just allowed 1.5 inches of water across the whole surface of your tank. That is a lot of water. Could be half your sump. For me, it is much more than the return section my return pump sits in.
So, I put my emergency drain just a bit more taller than my normal overflow water level. If the water level in my overflow box goes up too high, say because something got in the valve, then I don't raise the level of my display tank before my emergency drain kicks in. All my difference in water level stays in my overflow box. Does that make sense?