I clicked on the link, but the sound cut out after the first minute or so. I've learned over the years that making the HST's a little larger than the true finished block size, and then cutting (sizing) them down, results in the most accurate HST's. It might take a few minutes more, but the size is perfect.
I just start with two squares (oversized), draw a line on the diagonal, and sew 1/4" from the line on both sides....cut on the drawn line, open, press, "cut to size" and voila!! They're done.
Just my opinion.