When you say finished block, are you including the seam allowance? So the ending size of the block is 8"? If yes, the technically "correct" starting measurement for the squares is 9 1/4". To leave some extra to square up the blocks , start with 9 1/2. In other words, start with squares 1 1/2" larger than the finished size of your block, finished size excluding seam allowances to sew the blocks to each other.
Here's a link to a calculator
http://www.patchwork-and-quilting.co...triangles.html