Win 21 slams would set the deal for me. As much as I hate to admit, Djokovic would also already be ahead with 20. If he gets to 20 he will most likely surpass Fed in weeks at No.1, will be ahead in masters, H2H, slam H2H, most likely at least equal in WTF even the surface distribution argument would not hold water here (even though for me this is irrelevant anyways a slam is a slam). The only thing where Federer would still be ahead would be total number of tournaments won, which is meaningless.