I usually use Screen.width/2 and Screen.height/2 but what if I want to offset a guiTexture from somewhere like the left edge of my Screen 50 pixels? So in this case if I adjust my Screen size in windowed mode it will most likely remain on screen.
Hmm, lets do some math. 
Center: X = Width/2, Y = Height/2
Center-Left: X = 0, Y = Height/2
Center-Right: X = Width, Y = Height/2
Center-Top: X = Width/2, Y = 0
Center-Bottom: X = Width/2, Y = Height
Top-Left: X = 0, Y = 0
Top-Right: X = Width, Y = 0
Bottom-Left: X = 0, Y = Height
Bottom-Right: X = Width, Y = Height
Correct me if I am wrong, thanks.
I feel dumb now. I guess I was just tired lol