One thing I discovered using this method was This gives you the size one on the 4 pinwheel blocks, then you put them together to make your pinwheel or what other shape/ pattern you want. I was trying to make a 4" pinwheel, followed the above for a 4" block and ended up with an 8" pinwheel once all pieces were put back together. So you want your "block" to be 1/2 of the finished pinwheel size. 10" pinwheel would use 4 5" blocks. (Just rough measurements as an example) When you are cutting up your square you are making 1/2 sq triangles not the pinwheel. The pinwheel comes from the 4 cut blocks re sewn.
http://www.moonlightquilters.org/tipHSTfav.htm
Is another site with sizes for the HST.
I'm still new at this but it was something I learned the hard way.