If you use 2 1/2 in. binding and sew with a 1/4 inch seam, the binding will not be full when it is turned.
I trim the back and batt roughly to leave some extra before I sew on the binding. Then after the binding is sewn on with a 1/4 inch seam from the edge of the top, I use a ruler and cut the back and batt 3/8 of an inch from the sewing line. This give a full binding, especially if I turn it to the back and sew stitch-in-the-ditch from the front. The seam is always the correct width.