What are the best practices for implementing a GUI and doing relative positioning?
Trying to do it, change values and see what happends.
Use screen.height and screen.width ;)
So for example a progress bar that could be 0% or 100% of the whole screen width would be:
screen.width*0.55 - with the multiplier being your desired percentage. Here whichever co-ordinate of the gui object we are talking about would be positioned at 55% of the entire width of the screen.
http://unity3d.com/support/documentation/ScriptReference/Screen.html