Since you think it was making the miters, I would guess your block wasn't completely square, so squaring up or blocking the block square would help. Or the seams are slightly off. I'm always struggling with my 1/4" seams. Otherwise you could use the method for rectangles.
Just a thought!
By the way, the sample looks lovely, and while the one corner is noticeable if you are clued in the look for it I don't think most people will notice.