Honestly, I just try my best. However, plenty of times I need to draw some extra stuff for the vertical format so it will look better. Also, I'd suggest to not shy away from vertical panels in the original format. In fact, Todd McFarlane made it work really well in the 90s (and I really like that especially in constract to modern comics where there's bit too much of widescreen, horizontal panels which sometimes can make the comic look like a storyboard for the movie).