I actually don't use an E screen in my F3, I prefer a P screen. This has cross-hairs dividing the screen up into quadrants, as well as micro-prism collar and diagonal split-image focusing aids.
I discovered my issue with crooked horizons while shooting for my college newspaper. My eye problems are the result of a seizure I had when very young, which left me with third cranial nerve palsy. This basically paralyzed the right side of my face to some extent, especially my right eye, which also moved out of alignment with my left eye. I've had several eye muscle surgeries to put it back into something approaching the right position, but it barely moves and doesn't dilate (it's closed down to a smaller aperture). The doctors also suggested that my left eye be patched for several hours a day over a few years when I was young, to encourage me to use my right eye. As a result of this, my left eye is very nearsighted and I do not see in three dimensions unless I really concentrate on focusing both eyes on the same point. All of this got me very interested in eyesight and eventually photography.I had this problem for years. Didn't really pay attention until someone commented about slightly crooked horizons. I went back and looked closely at past photos and realized it was true.
Looked in a mirror and realized my eye sockets themselves are not perfectly horizontally aligned. One is slightly higher than the other. Don't know if that's the cause. Maybe I'm like Dr. Frankenstein's Monster. Could also be an inner ear defect, I suppose. But my eyesight is fine.