Alas, the real thing proved much more difficult. Effective cropping of the photo makes the block for Jen look more accurate than it really is. The block doesn't lay perfectly flat, and there's an edge or two that almost makes it to 8.5 inches. You'll notice my test block uses regular quilting fabric
whereas Jennifer's block is constructed with batik for the two concave arches (dark purple and chartreuse), and I can't help but wonder whether the lack of give in the batiks contributed to my troubles. On a positive note, I think the block looks way better in real life, where the colours are much richer than they appear on the screen.
The finished block and leftover fabric will hit the mail tomorrow.