What I have always done (even for the 90 degrees) is to sew both seams up to the 1/4 inch seam allowance with extra fabric hanging over (unsewn)... Then I turn the quilt to right side up and leave one straight/flat and use the top one and fold it to meet the other on the outside. In a 90 degree miter this is a 45 degree line. But I don't see why it wouldn't work for a differing degree change. You can basically line up the two points to create that line (and not really worry about degrees)
I then press it very well so I can see the line well. Gently open the top piece while keeping the bottom still and in place. Then I pin across the seam from the outer point to the inner. And sew....
Hope this helps!