Oh good point, I actually used to use portrait mode too when I had a single display but since I've been using dual displays it seems a bit more difficult to do lolThat again depends on the orientation of your picture, sensor, or display. I frequently use my computer's main monitor in portrait mode. (When I first started with S.E, I would always run it in portrait mode.) If you take a picture in portrait mode, you would want that FoV value for the horizontal axis.
But I know what you mean — FoV should always be the smallest axis. (I believe S.E. always used the vertical axis, even when in portrait mode.)