Mayweather gains a #10 ranking at welterweight according to Saddoboxing.
He was the lineal champ when he retired and had a win against the #2 p4p guy, because of all the circumstances surrounding the fight and the WW division as a whole #10 is a fair rank. If he wants to move up, he needs to beat someone ranked higher (i.e. a Welterweight), simple as.