I use the measuring cup technique as described above as well. I personally don't think you need to use live sand as you already have established sand. The new sand will be "live" very quickly. I would also suggest you don't add too much at once and also gently stir in the new sand with the existing. My worry has been that a layer of new sand might trap aerobic fauna at the surface of an existing bed....my opinion only with NO supporting scientific evidence
