water height in overflow box

[Chad]

I have not drilled anything yet.. I am trying to understand this first before I go drilling my tank.

I will be used two 3/4" drains. My idea was an 90 degree elbow down with slots cut in it so it will butt up against the bottom of the overflow box.. also providing a screen. The other drain will be 90 degree elbow up with a small pipe that extends I figured, 1/2 an inch down from the top of the overflow box.

But, since I don't understand what will control the water level in the overflow box, I am not sure if I will have enough height. now, I get the gate valve will back up the drain so it is even, thats no problem.. got it! :-)

If someone can give me this in a K.I.S.S. format I will be very happy