OP, first thing it's important to understand that the term "clean" refers to a credit report that has no negative items. Since you have a 30 day late payment, you're file wouldn't be considered "clean" using that definition. That being said, FICO scoring in terms of score card assignment may overlook an isolated 30 day late payment and put you into a clean bucket rather than a dirty one.
As for score gain potential, do you know what your scores were before the 30 day late hit and what they were right after it hit? I've never tested a lone 30 day late payment hitting the 2 year mark, but I'd guess you could see 10-20 points or so, just shooting from the hip.
