I have had pretty good results with the Canon 77 Close-up Lens 500D, albeit mainly with the 70-200/2.8 at longer focal lengths. With the 24-70 you will not get much additional magnification, even at 70 mm. I believe you would be much happier with a dedicated macro lens. I got very good results with the excellent Tokina AT X 100 AF PRO D Macro lens. The current price of this lens is about $450.