This is a really gorgeous quilt and the border will be gorgeous as well. I congratulate you on determination and you will get it to work. This does look like a tough border to add, everything throughout the whole quilt has to measure up perfectly, with no stretching of any sort. I think, this type of border would be easy to sketch out of graph paper for a quilt, but the actual sewing of it into a quilt probably requires some fudging, because any deviation from perfect will end up showing. As other's have said no one will notice, except you the adjustments you make and will be gorgeous.