You have the position formula, but what would the size (width/height), of the control, formula be? I don't want it to be the same absolute size on every resolution. I want it to scale proportionately with the resolution.so that you won't have more clean space but so it looks the same on every resolution.
As far as I can tell using %'s for position does the same as what you're saying as the % is always based on the top/left corner of the parent control. Results in the same positioning it would seem, and for me %'s make more sense in my head. The question still remains on the sizing of controls though and how to scale them proportionately across all resolutions using a base resolution that the values were designed against.
I'm making an editor to design my GUI screens. In this editor you give a default resolution that you position all the controls against. In the editor the positioning will be in pixels, but when I save the positioning to file I'm converting them to %'s. The game will read the %'s against the resolution and position based on that. This way everything is done automatically for me. I'm going for a WYSIWYG editor meaning the position & size of the controls will always look that exact same on every resolution giving the exact same look across all resolutions.