The only rule is there are no rules. If it doesn't bother you that the back binding is wider and your corners miter nicely, go for it. If you want it smaller, sew a deeper seam but only if it means you will not loose any of your points if you have them on the front. Also folding it double again will work (may have to work with the fullness at the corners), or you could take it off and re cut it. Sometimes there are no right and wrong ways, just different ways! Whatever you do, just do what you will like so when it's done you are happy with it. I like the stripes, very cool!