When doing the 7/8th, you REALLY REALLY have to be careful and sew a scant 1/4" seam. You also have to press carefully because you have no leeway on the finished size.
That is why I use the finished HST size +1" plan. I don't have to do a scant 1/4" or even a perfect seam because I will trim the HST at the end. A little more work but quicker and more accurate for me in the long run.