Your eye should fall on the middle of the top third of the artwork if you are looking straight ahead at it.Of course, we are different degrees of tall so that space will change depending on how tall you and yours are. Mostly, though, you don’t want to have to look up or down to see the work. As for that space above your sofa, you don’t want your head to bang into the frame when you lean back. That means that the depth of the seats on your sofa will affect how high you should consider hanging the work. Over a buffet, you need to consider if you will have other things on that furniture. If you do, they should really be off to either side. For a piece that is close to the same width as the buffet, the bottom of the artwork should be about 5-6 inches up from the top of that kind of furniture.
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The purpose of a frame is to focus your eye. To focus the eye of the viewer, so that your peripheral vision isn't distracting, doesn't distract you from the content.