I drill 4 round opposing holes starting just 1/4" from the bottom of the bell. I use the same size standpipe as the return tube. The suction of the water going down the standpipe creates the draw. The round holes break the siphon. Half holes don't work for me. I am not an engineer, I just know that I have six of them running flawlessly. Pictures of mine are in my thread, rfeiller outdoor garden construction.
You notice I don't have a photo in my avatar either, I have little patience for jacking around with that stuff.