My photopolymer platemaker tells me to set the width of my file to 13″ and fill it up from there, because no matter what the plate is going to come out on that width. I rotate and cram in as much as I can to set the plate up as short as I can using that width, and have never bothered with more than a few pixels of margin (enough so I can visibly see that the image is not touching the edge). Never had a problem, and never been overcharged, but I do need to keep some items on hand to fill in the space when it’s there, because I’m paying for it anyway! Good luck!