I think most people do it in 'portrait orientation.'
I was a commercial diver for many years, that means working blind most of the time in rather dreadful conditions. The more body contact you have with an object the more tactile sensing and orientation you maintain.
With the holder in portrait orientation lying flat on a table in front of you angle it upwards 45 deg. flap at top, now bring the bottom (dark slide end) in to make contact with and resting on (depending on the extent of beer consumed on average) your tummy. Right hand holds the film with notches upper right and *I* use my left hand to keep the flap open with my index finger while my thumb rests just below the film track opening and helps guide the left edge of the film into the track.
Better sensing, orientation and leverage.