I almost always use a 5" binding, folded in half and then sewn on. The trick I was taught is to whack it off at one end. For the other end, lay it on top of the trimmed end and measure the full width of the binding longer than the first end for the overlap. So for me it is almost always 5". When you put the right sides together and mitre them together you end up with the right join.
Hope that helps. It always works for me.
Tara