[Snapcase]

Your assumptions about creating a controls layout in PyXBMCt are basically correct. This is more of a trial-and-error game until you get your layout right, and there are no specific guidelines.
I said is like a million times and repeat once again: PyXBMCt is just a thin convenience wrapper around xbmcgi Window and Control classes. It provides a grid layout instead of absolute pixel coordinates and a set of default textures to decorate your Controls, because most of xbmcgui Controls do not have their own textures.
By default, xbmcgui.Window and WindowDialog operate on a coordinate grid of 1280x720 pixes. This is a purely virtual coordinate grid that do not depend on you current skin or display resolutions (and they are different things). Controls are placed and sized in absolute pixel coordinates from the top-left screen corner. Instead of this, PyXBMCt provides a more convenient grid layout and calculates actual Controls coordinates in absolute pixels "under the hood".

Thanks very much for your help - and for the add on in the first place !