I have also pieced my cut offs into pinwheels and will use them in the border. I only have the medium and light ones, as I didn't plan to do the others as covered corners. I personally struggle with covered corners and can never seem to get them right. If I do this pattern again, I'll cut triangles and a smaller square to start with and sew them together. I never have problems getting the correct finished size when I do that. I know for most, it's the opposite!
I am on the last two rows of assembling the field, and then I can start on the borders. I have the pieces for them sewn, but still need to trim the smaller pinwheels down to 2.5".