This is how my mind works out the math.
Normally, you take what you want for a finished size (2") and add 7/8" and cut two blocks, mark diagonally, sew on each side and cut to get two squares. I always add and extra 1/8" so I would start with 3" blocks.
In your diagram, you basically have 4 blocks. So 3+3=6 and I would start with a six inch block and follow your instructions.
Finish 3" would be 8" block
Finish 4" would be 10" block
Looks like the formula would be finished x 2 + 2 the way I do it.
And then I would square up.....however, there is a tute on here for making HSTs that I love, and have been using that method...no squaring up! You sew two strips together on both sides, and use a triangle ruler to cut the blocks out. They come out perfect with little waste.
Diane