I've done this before too. Hope this helps!
Ok, I'm not sure what size HST you are wanting to finish with. If you need 8 (2inch) finished HST, you would want the starting squares to be 5 3/4" x 5 3/4". So, in other words, start with 7/8" larger than the finished HST should be when making only 2.
8 x 3 inch finished = 7 3/4 x 7 3/4 starting squares
8 x 4 inch finished = 9 3/4 x 9 3/4 starting squares
8 x 5 inch finished = 11 3/4 x 11 3/4 starting squares
8 x 6 inch finished = 13 3/4 x 13 3/4 starting squares
PM me if I can make it more clear. :)