I usually trim the batting a little before I put on the binding, but leave an inch or so of batting and backing. Then I sew the binding onto the quilt with the usual 1/4 inch seam. Now I put my ruler on the sewing line and measure from there. Usually 3/8 of an inch will fill up a 2 1/2 inch binding. If your binding was three inches, you need more on the outside of your sewing line. By measuring from the sewing line, the binding always has the same width to cover all around the quilt.
I would never do all that for binding. I turn my binding until the edge of the quilt layer stops the turn. That will be the width of it all around the quilt doesn't matter what width it is as long as it's the same on all four sides. I have utility quilts for the grands that are years old, washed many times and the binding hasn't frayed at all. The first few quilts I made with full stuffed binding as per the book, that binding is fraying and the bindings were made from quality fabrics. I would prefer my binding not to fray instead of being made to fit a rule.