With all due respects to Mr. Malty, I think it over estimates the amount of yeast needed. What does it use to determine what the correct pitching amount is.
I'm not advocating grossly under pitching, but it seems that there is an optimum level of under-pitching that brings out the best in the yeast. I have read a number of reports from brewery's that deliberately underpitch to bring out more yeast character in their beers, I think 'Farmhouse Ales' also advocates under-pitching.
However; I think a single pack of Notty in a 1.080 is pushing it, but you should be OK. There are a couple of problems with severely under-pitching: Stressing yeast and a stuck ferment, and creation of fusel alcohols.
Adding a second pack a couple of days into the brew is not going to do much. he reproduction phase is over.