I used to make jackets for my late mother-in-law, who had a dowager's hump (what an ugly description of a spinal curvature!)  I draped a muslin on her body.  The main feature was a back yoke, with possibly a shoulder dart in it--can't remember.  At any rate, drape the yoke over the asymmetrical shoulders, and mark a level seam line where it starts to hang free of the body.  The back, which can then be cut on the straight grain, hangs from this seam.  Since the yoke will be somewhat on the bias, interface it, so it will support the back of the jacket without sagging and distortion. Hope this helps.