You could probably ask 10 people this question and get 5 different answers. The one in the picture is at dead center and that's where I'd mount mine if I were doing this style. You can check out dacolson recent builds and see where he put his because he has had good results with his cookers.
Height of said collector is generally the same as the diameter of the stack. The creator of the plenum style (Franklin) installs food grates dead center.