Make your pinwheel half-square triangles finish at 3" square.
Sew them into the pinwheel that will be 5.5" unfinished.
Cut 2 (background squares) at 5" square; subcut these on ONE diagonal to make 4 triangles.
Sew the triangles to the sides of the pinwheel block.
Square up to your required 8" block.
Doing it this way has several advantages:
1. The 5" pinwheel is a measurement you can easily divide to work with.
2. The 2 background squares cut on one diagonal will give you straight grain of fabric on the outsides of the finished block, preventing stretching.
3. The pinwhell will "float" within the on-point block, preventing you from losing any of your points.
Jan in VA
Well, you certainly can't see that!!!!
The words and arrows in the TINY graphic point to the overlap of the setting triangles which allow the pinwheel to "float".