E.g. if you have a dialog with width 400,
And a button with with 100.
And you want to use left alignment on that button, but have it in the middle of the dialog.
Then the width offset is ( 400 / 2 ) - ( 100 / 2 ).
Well, that's pretty easy math, but as long as you keep these numbers inside your head it's not hard to design a very accurate UI on paper before porting it into SC2.