The second pass will cure many ills (suckback) but be sure you've still got that purge gas going strong. Since you're not paying for the gas, I'd say 20cfm is within reason. I was trained with too little gas because they were paying for the gas. Previous posters seem right on, to me. Try that knife edge, by the way, and experiment with progression (six to three, nine to twelve - anything that keeps the gap from closing. Pretend it's late, you're tired, and you've just done seventeen of these things.