To find the fov of the elo sight, you compare the screenshots. If you know the fov of the hipfire, and you know the amount zoomed in (from comparing the screenshot), then you can find the fov of the elo sight.
You place the two screenshots into photoshop, or an equivalent tool. You place the ADS layer above, reduce the opacity to 50%, and then reduce the size until it blends in perfectly with the hipfire image. The resize percentage is then used in that formula above to find the fov.
Your screenshots wont work though because you moved the mouse and aimed higher up when you took the ADS screenshot.