I use the 20" square up ruler and another long ruler lined up together on the quilt edge. Square up the quilt edges and trim. The second ruler helps me make sure the sides will continue square.
Binding is cut at 2.5". I sew the binding on the back of quilt with a 3/8 seam.
Like ckowl, I found that 3/8" is the magic number for having binding equal on both sides.
I have also used 2.25 " binding like Quilt Nan.
All the methods work due to the math.
Make a sample and try the different widths of binding with different seam allowances. You will find your magic spot.