I think you missed how flipping the B screw around is actually a very simple solution and will solve your problem. Since the head of the screw is wider, it will better engage the drop-out. Normally people do this because they need more extension than is possible with the existing screw (or they buy a longer screw).

I have the same problem you're having on my bike, except it happens at about 70% before the screw basically pushes by the drop-out. Just flip it around and you'll be all set. To adjust, rotate the derailleur back, do a couple turns of the screw, and let the derailleur rotate back in place. Check your setting, and repeat until you have it right.