Originally Posted by quilt addict
It depends on which math you want to do. You will want the pinwheel blocks to have a dimension that will be even with the sides of your center piece. Currently I don't think it will work because 18" and 20" do not have a same divisor unless you are going to do 2" pinwheel blocks. If you made it an 18" square center then you could use 6" pinwheel blocks on each side.
Does that make sense?
Good luck.
good point, OP would have to put a little sashing strips on her FQ to get it up to a nice evenly divisble size and add her necessary SA as well. Or she needs to cut it down a smidge.