That method isn't particularly special but she did offer a wee tip that for me, made all the difference. Listen carefully near the latter part of the production, when she tells you not to stitch right on the marking line -- but a hair inside it, so that your seam allowances will come out right once you've pressed your seams.
Yep -- that was it. I've now made fifty (count 'em!) -- fifty more units and every single one is the desired size: 2.5 inches square! Am I pleased, or what?! (And yes, I now have a total of ninety correctly-sized units. Only 135 or so to go, right?)
Oh yeah, and while I'm plugging useful items...
If you are using the 3.25" squares method of making these units, you'll find marking the sewing and cutting lines easier if you have one of these little things: a "Quilter's Quarter" -- also known as a "Quick Quarter", from Quilter's Rule. I have an 8" one and have used it for years (got it at my Local Quilt Store). The only time I don't make HSTs without it is if I am using triangle paper (e.g. "Thangles") with which to make masses of HSTs in one fell swoop. But I've never tried QSTs with triangle paper. This wee ruler makes it easy to mark and sew without paper. Enjoy!