for a GUI box(or Label) 600x600 size, you can say that 600 = Screen.height*0.6f so it will be
new Rect(Screen.width*0.5f-Screen.height*0.3f,Screen.height*0.5f-Screen.height*0.3f, Screen.height*0.6f,Screen.height*0.6f)
the start point of your GUI box is at top left corner so the center of it , will be in the center of screen for any screen size**
better than that is
//adjust your values here to select the desired size for your Box or Label
//in Play Mode you can see directly your changes after saving script first
float boxWidth=Screen.height*0.6f;
float boxHeight=Screen.height*0.6f;
//Now with the given values make the dot for draw start potision(top left corner)
float startBoxWidth=Screen.width*0.5f - boxWidth*0.5f;
float startBoxHeight=Screen.height*0.5f - boxHeight*0.5f;
//and finally
new Rect(startBoxWidth,startBoxHeight, boxWidth,boxHeight)
//this can be done also for GUI.Label ,Box , Button etc.
First, Screen.width - Screen.width/2 is the same as Screen.width/2…
Second, the position of a Rect specifies the position of its upper left corner. So to center a Rect on the screen, you’d have to subtract half of the Rect’s width from the position.