My calculation is correct?

yes, if  you Join  for training till 30Oct , then your final will be May 2014 , if you join after that it till be extend by 6 months

Yes thats correct but then u might face a shortage of leaves bcoz the last six months of articleship are counted after considering the extension due to leaves. So its adviseable to join as soon as possible to be able to avail of leaves for your final attempt.