When the focal length is half of the negative diagonal, the viewing angle will be 90 degrees, corner to corner. There will be considerable fall-off in illumination in the corners, but some photographers are happy with even wider coverage in pinhole photography.
A sport finder of the same size and proportions of the negative image can accurately frame the view if the eye is centered at the focal length behind the finder. Accurately positioning the eye is tricky. Adapting the viewfinder from other cameras may be more difficult; they rarely cover a really wide angle.