I dunno if this helps, but I tend to draw my full strips at 350 dpi (yeah, it's high, but I can't -stand- 72 dpi lol especially since using 350) and at their regular size, whether or not it's 940 pixels (which it isn't lol my strips are much larger). When I convert them to vertical, I do all the formatting, then I just go to "Image Size" in the Image properties panel or whatnot in Photoshop and shrink it down to 940. I don't know if that's the "professional" way to do it, but it gets the job done and my comics still keep their sharp quality here.