I square up my quilt having the backing, batting and top all the same (I don't make the backing/batting larger). I lay the raw edge of the binding right up to the raw edge of the quilt and sew 1/4". When you flip to the back to sew it down it should be fine. If you find you have excess when you flip it to the back then cut your binding strips 2 1/4" instead of 2 1/2".
Hope this helps.