Auto- scale Gui button

Hello everyone.

I have one question, I did button and I set positions but this button behaves differently and is in different locations depending on the screen resolution.

How to do that was all the time in a fixed position? Regardless of the resolution?

if(GUI.Button(Rect(Screen.width /2 - 100,Screen.height /2 - 100,250,50), "Play"))

if(GUI.Button(Rect(20,20,250,50), “Play”))

This is a simple position of GUI… Try next positions for example:
…20,300…
…300,20…
…250,350…
(Top, Left)
And many many more GUI positions… I using this:

if(GUI.Button(Rect(Screen.width /2 - 100,Screen.height /2 - 100,250,50), “…”))

When I want middle center the GUI…