Rumor has it that Buchanan also got a raw deal over in Japan and was robbed in a title fight against Ishmatsu Suzuki a couple years after the Duran fight. The force of Duran's punches actually cracked Kenny's protective cup. He jokingly told Duran recently when Roberto was visiting him in Scotland that he remembers that fight everytime he goes to have a pee. Funny quip but probably not really funny at all when you think about having a permanently damaged testicle.
Around 3-5 seems reasonable. Buchanan was certainly competitive in most of the rounds he lost. Infact, a very competitive fight for as long as it lasted. I'm sure you agree.
Yes, apart from round five where he got battered. It might have been the twelfth (or eleventh) where he had another rough ride.