This is the reason I love having Electric Quilt 7. No more math problems. I would look to Google for information:
https://www.google.com/search?q=HSTs...utf-8&oe=utf-8
I did this design for a friend who wanted to see how to make one. You need to state what size of quilt you want including borders and then that will be the size of your HST. Remember the 7/8" you need to add to the size of the finished block size. Hope this helps.