Hej Annika

As far as I can see it, you divided the whole block in fourths..
So you made the cut halfway between the middle line and the rim.
You should have the whole block divided by three. so if your pinwheelblock has a little more than 12" I'd cut it down to 12" and then 12/3=4 => so you have to cut 4" from the rim or 2" from the middle line, whatever works best for you.

Happy new year