Pillow covers normally call for a 1/2 inch seam. If adding a zipper that side would be turned under 3/4 inch seam. If you want a plump pillow you need to make the cover 2 inches smaller than the insert. If you have not pre-washed your fabrics I would go with a 19 inch finished cover to allow for shrinkage.
To prevent "dog ears".....measure each corner down a 1/2 inch & 3 inches. Draw lines from one corner to the next. The lines will cross. This site shows you:
http://playfulstitching.wordpress.co...illow-corners/