I would actually setup where the D is, but its up to you and what you think is the sweet spot. But it will be in that general area, right in the center, about halfway back from the stage...
From each PA speaker, left and right, draw a equilateral triangle (60deg angles) with the one point at the PA and the other two points projected out into the room. You want to stand where the two triangles overlap...
At a show where there is a ton of dancing/moshing/craziness in the middle, you may have to stand off to the side and do a stack tape (not preferable, especially in cases where PA is run in stereo), or you'll have to stand in the center further back.
You could tape from one of the round tables in the back of the dance floor and run you stuff under a jacket or in your girl's purse or something...
Terry