Everyone has their own way of doing HST's. I like doing them the same as in that tutorial, except I like to make them oversized, and then trim them down to the exact unfinished size.
So if you're CUTTING a 5" square from the focus fabric for the center square, you should cut squares for the corners at least 6" . Or larger.
The exact way of measuring is to take the unfinished size of the HST block - in this case 5" - and add the .5" diagonal seam allowance plus 3/8" for the weirdness of the angle. So the exact measurement for the square would be 5 7/8", but who wants to deal with that.
I love the Churn Dash block. So classic. I've had a quilt in the works for years - Amish Aliens on Acid. I named it right away but it's nowhere near done. :roll: