Sound like we are all a little confused as to what you are talking about. I suspect it is not HST but as you mentioned Square-in a Square. Do you want your center square to be on- point or do you want the block to be square? If it is to be square then I would just do sashing around the block. If you want the center block to be on point then you do need to make "setting" triangles for the corners- this is an entirely different calculation than for HST. Check the internet for " calculations for setting triangles" to see how to make these. This is not the "7/8 rule" that is used for HST. .
Here is Bonnie hunter's version:
http://www.stcroixquilters.com/setting%20triangles.htm