I have tested my ability to judge the vertical, at home in a quiet room, standing on a chair that was not moving. I can do no better than plus or minus 1 degree. An error of plus or minus 2.75 degrees corresponds to a 1,200ft error on the ground when viewed from a plane at 13,000ft agl. I am not at all confident that I could achieve this accuracy from a noisy, moving airplane with other jumpers moving around.