Making the graphic square is more important than the actual PPI. If 19 PPI gives you a square graphic, then it’s good. It was fun to weave, my first attempt at using cottolin.
The draft actually calls for 18 EPI. I only had a 10 dent reed available, so I went with 17.5 EPI.
Loom width 18.89 inches in the reed, 17.5 inches off loom, and 16.5 inches finished. Loom length 34.75 inches off loom(before hemming) and 29.75 inches finished and hemmed.