I used to really hate binding quilts. Corners came out cupped or dog eared. When I jumped into quilting for hire I had to get it right. Most directions allow for 1/2" finished without problems. Wider than 1/2" and I was in trouble. So I had to sit down and figure out how to consistently turn a corner with any finished width binding.
I am going to assume that readers know how to fold a corner. The point I want to make is - do you know where to put the fold. Most directions tell you to line it up with the edge of the quilt.
I do mark the finished perimeter+1/4" and make 1/4" corner marks so I know where the finished corner is. We all sew to within 1/4" of the corner and back-tack. My little 1/4" line is my guide to stop and back-tack.
To put the right amount of fabric in the corner I measure from the seam to the fold of the binding. That measurement is transferred along the seam line of the binding and I mark a dot. The binding is folded as usual and the dot goes right at the corner, using the corner mark. The images are of a 1" finished binding, cut width is 4 1/2". I begin sewing the next side at the fold. Cut the quilt layers after the binding is attached.
Back to the workroom........ Cheryl