Generally I mount square prints almost identically to DWThomas, same image & frame size except with no white border around the prints, matting to the edge of the image.
How much I offset from top & bottom (which I think is your question Ralph) depends on how the image is going to be displayed. My exhibition work is a mixture of negative formats 5x4(10x8), 6x6 and 6x17 (all full frame) in the same 20x16 frames so I spent some time doing visual comparisons of image size and in frame positioning with each format, first on the PC like Ralph's drawing, then made actual prints. It works out that the vertical centre of each image size is constant for landscape format, because that seems to work best visually.
More recently I've been selling some square images in square frames, initially because a local store (UK) has some very nice square frames at bargain prices, but they great when finished and hung on a wall.