Having been trained in the visual arts, I would say the idea placement would be to align the center of visual mass of the sporran with the rule of thirds.

In other words... take the open area of the front of the kilt that is seen (don't look at the part covered by the belt), and divide that into thirds. so you have two horizontal lines running across, one is a 3rd of the way up from the bottom, and one is a 3rd of the way down from the top. Place the sporran on the top line. Not the top of the sporran or its bottom or its center, but its "center of visual mass". Which means you have to take into account its shape, it features, etc. In short... where it looks "right".

Having said all that, I place mine so that it meets the above, but then I adjust it so that when i have my keys and wallet and cell phone in there, it will not rack my boys if I were to start to swing.

(i am typing this really late... i doubt this is all coherent)