I have a bit of experience making throw pillows. I always make the pillow form 1" all around larger than the finished top. This way, when the form is inserted, it will push on all sides and take care of empty corners. Use a 1/2" seam allowance for the top and in your case, the top should finish at 17".
One other thing I do is grade my corners, i.e., starting about 2-1/2 to 3" for the corner trim toward the corner removing about 3/4" when you reach the corner. Do this on all corners. I actually have a template that I use just for this purpose.) I do not like to see empty "rabbit ears" on pillows. Just me.