You did it right.
I like the look of a pinafore on adults or girls. For a long time I searched for a pattern for an apron like the one worn in "The Color Purple". Problem is, I collected a lot of patterns during the searching, yet have not completed any of the aprons. I think if you have any pattern that features a yoke, front and back, the body of it could be adjusted in length and width. I have a pattern that I intend to put to good use, called "My Neighbors Apron". It is much like the Pinafore Apron that you posted the link to, only the yoke is bigger and the armhole deeper, running almost as deep as the waistline.