From 13,000 feet, a 2.75 degree error in judging the vertical leads to a 1,200 foot error on the ground!

How well can you judge the vertical from a moving aircraft?

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.