This is a very good question. Hems of a garment with a slant have to be treated so they fold up to the slant. What I do is tape a piece of paper wider by 5 inches than the pant at least 6 or 7 inches long onto the bottom area needing the hem.
Then all you do is fold that paper up just as if you were hemming the shorts for a cuff. Cut along the side seam following the shorts line. That will give you a perfect hem on a slanted seam. This will look very odd when you cut out your fabric but it turns out really. It helps to put in your seam allowance first. How long does it need to be?
If I'm not making a cuff I like a hem on shorts that will give some support to the area and keep them smooth on my leg. A 2 to 2 1/2 inch length on a short is perfect for me. So then you need a 1/2" fold over so I would Add a Three inch length to this short.
For a cuff you must take into account the extra needed to fold forward on the outside. I hope the page I put above will help you. It's from my favorite sewing book. Nice clear drawings and plenty of choices on how to do a project.
Yes we welcome questions and Ann will always look them over and add anything I've missed and visa Versa.
Ya know Sally, the more I look at that pattern the more I think Fonnell is closer to your answer. Below is a photo of a pattern with a similar cuff treatment to yours:
F. The dotted lines are fold lines, there are three. Could the angle of the hatching be to help you fold it inward, rather than outward. As I look I can see the cuff apear. I've checked all the guides and seperate made facings are marked with hatch marks and ST. On the knit garments the hatches show a folding in for self fabric facing and there are no St (facing) markings