Always look for corrections on the publisher's website.
http://www.martingale-pub.com/store/...ons_search.php
The authors should have it all as well, but.....
And there are none for this book. <shrug> I find it hard to believe as pretty much every book, despite everyone's best intentions, have some small error or typo. It's the nature of the beast.
I think the way to approach this is to decide what size you do NEED based on the other pieces. It's essentially a 9patch.
So what size completed rectangle do you need to fit with the rest of the pieces to make a square block?
4"x8" So make it work.
Actually, I'd probably PP it. But then I'd probably be pp-ing the entire block.
If I were going to do it without a foundation, I would do a version of the snowball method, adjusted for the odd shapes of a half rectangle.
If you draw a rectangle around the diamond shape in the template you have, then you can easily add the correct seam allowance.
If you cut the diamond from fabric according to this new template, you can cut two oversized rectangles for the corners. Test this with scrap fabric.
Cut them in half, and attach to the diamond on the 4 corners. Then trim the rectangle to the correct size.
A bit convoluted, but then there's no "coming up short" in the final piece.