The aspect ratio of the ScreenGui will need to match that of the SurfaceGui.
So if the ScreenGui is scaled to UDim2.new(0.4, 0, 0.2, 0) or otherwise an aspect ratio of 2:1, then the SurfaceGui will need to be sized to scale in which its aspect ratio is also 2:1, for example UDim2.new(0.3, 0, 0.15, 0).
The aspect ratio is simply the ratio between the length of the x-axis compared to the length of the y-axis (be it as a scale or pixel count of the window’s resolution).