Some painters seem to get away with it, and as shown in these pictures it really gets difficult when you're stretched out. Maybe the pro's have a way to better know the angle of the gun on horizontal panels, but it seems to help me if I extend my index finger, and use the middle and ring fingers on the trigger, then use 75% overlap with a little faster speed when spraying the horizontal panels.
Many years ago when Iwata released the LS400 Super Nova they had a video demo of that gun at the Sema Show, and the demo guy was holding the gun with his index finger extended. There was nothing said about it, but at the time I was having a problem with striping and picked up on that.
I stole these pictures off the internet.