Keeping custom widget designer centered

Hi,

I'm developing some custom widgets in SF 8.2.  I've established that you can set widths and height using dialogBase.setWndHeight() and dialogBase.setWndWidth(), but how do you keep the designer centered after the width has been defined?

I can re-center using javascript in the refreshUI method but when I click on advanced, the designer positions to the left, and then when I click back on simple, the designer positions to the right.

Is there a javascript method/event i can use to re-position the designer in the center?

While I'm typing...how do I add the 'add content' icon in default widgets as below:

http://i.imgur.com/Sm6B5Ss.png

I hope what I'm asking makes sense!