I strongly encourage using uigridlayouts. Everywhere. Every componentcontainer I build starts with a grid, and every app has a grid at the base of the figure. Using grids makes nearly all conversations about size and position irrelevant.
To your specific question though, the setup method executes before any input arguments are processed. To obtain the parent of the componentcontainer, you need to do it in the constructor, or the update method. In either of those two methods, a simple obj.Parent.Position will give you the information you want.