I've just sewn 2 DHG2 bolcks myself starting from 10" squares I had cut from yardage. I was very careful with my 1/4" seam allowance, and when I united the HST together along their bias sides, the block measured 12 3/4". Divided by 3 means the cuts should be made every 4 1/4", or 2 1/8" from the central seam.
I'm confused with the following step, as my final block comes up at 11 3/4" and not 12". Actually, if you make the math, you are now cutting a 12 3/4" block so that you'll add 2 seams in between. This is, 12 3/4" minus 2 times (2) 1/4" seam allowance results in 11 3/4". If you want to obtain a final 12" block, you must calculate scant seam allowances in this last stage of the process.