Well, I don't have spray starch, so I'll have to wait until tomorrow (almost 10pm here; if I had the car, I'd run up to the 24-hour Safeway...)
In the meantime, I've measured the eight finished diamonds; NONE of them are true. The closest one to "true" is 13.5 x 12.5.
Four of them are 14 x 12.5 (ugh)
Three of them are 14 x 13
So, I *think* I need to shoot for 13 x 13, right?
Or am I just um... screwed? (sorry for the language...)