Disappearing Pinwheels and Math!

I am currently playing with Jenny Doan's Disappearing Pinwheels. I love the idea but don't like the size you get from a 10" block. 
 

  I tried with 6 1/2" half-square triangle and got 11" unfinished blocks. 
 

  Next I tried 8 1/2" half-square triangles and got 14" unfinished block.
     I want a 12 1/2" unfinished block. So now I am going to try a 7 1/2" half-square triangle.
    It worked! I cut the squares at 8" and resized the half-square triangles to 7 1/2". Then made the pinwheel 13 3/4" which gave me a perfect 12 1/2" block.  I love math when it works! The pinwheel you cut must be 1 1/4" bigger than your block size. I think!