Welcome to the board. And glad you "jumped in with both feet". I think you'll be just fine. 1/4" seam is good and like others suggested, a small border around the top would be fine. OR:
since you're making an entire pillowcase. Sew extras to the back where you do the fold over for the closing. This is a good tutorial to watch for a 16" pillow. It's how she does the back that will most likely be exactly what you need to finish the cases.
https://www.youtube.com/watch?time_c...&v=4LJnPEvjdDo