I find that most problems with good mitre corners is the seam width doesn't remain constantly to the place where you stop. The feeddogs tend to pull the quilt about the last 1/2" or so making the seam narrower. There is an interesting video, I believe, at the McCalls web site where Patrick Loose shows a place at the corner where he trims. I don't have the site but you might be able to search his name. It may be Lose. With a binding cut at 2.5" I use a 3/8" seam. It tends to fold over better. I use a lot of borders on my quilts so I can do more than 1/4" seam.