I did it the wrong way and used single cut squares (Figure A) instead of double cut (Figure B) as setting triangles. The hypotenuse (that goes to the outside) is on the bias in Figure A, but on the straight in Figure B.
It does take some imagining, but draw an imaginary red line along the straight edge (or put a pin, if you need to do it in real life), and follow where that ends up on your quilt.