Ok, so i have this gui, the button is {0, 250},{0, 50}
What is the middle position? It is offset, you can test here.
http://www.roblox.com/games/255925361/Super-Meat-Blob-NEW-PROJECT
The middle position would be UDim2.new(0.5, -size.X.Offset/2, 0.5, -size.Y.Offset/2)
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size = size of object you are positioning, and not the parent GuiObject's size.