I'm not familiar with this pattern but I would suggest using your seams to square up. If your strips are not sewn evenly, you're going to have issues. I think most of us have a tendency to have the ends of long strips wander a bit and this might be what's throwing you off?? Could you fold in half in the center; use some pins up/down through your seams to make sure they are straight? That might get you to straight edges on both ends?