[Drunken F00l]

Your image must be 1024x1024 without any scaling, otherwise you will run into issues like you said. To do this, run in 1280x1024 resolution and just clip everything to the right after the 1024th pixel when saving to jpg (canvas size in photoshop with topleft as the anchor).

You must also use the scale, pos_x, and pos_y that shows in your console just before you took the screenshot. You must be standing still (preferably noclipped) so you get a steady reading.