I've had a look at the pictures of Trevethy Quoit, closely, and this isn't the best example to illustrate my theory with. The aperture, in the capstone, is so irregular as to make deriving a consistent measurement, for the length and breadth, very difficult. There are other holed stones which are better examples but they are not securely dated. They often get identified as some kind of gatepost.

The theory is that the 4:3 ratio of viewing a scene was invented/discovered by prehistoric people. We think of sun or moon light coming through the stone, illuminating whatever is beneath, but it must also be possible, with the correct posture and monument accoutrements, to use the aperture to watch the sky, or a segment of it, go by. Which is what we do with windows, in walls and on our desktops.