for that amount to last for 13 years, everything has to go according to plan without the slightest hiccup. life is never so predictable. there are many perils along the way, just when he thinks he can coast smoothly. an syt may show up, and he may fall crazily in love. a sudden medical condition may warrant high expense. a sinkie scam may be so convincing he parts with a huge sum. an online nigerian or belarussian fraud may trick him into cutting a big check. relatives, friends and even family may need monetary assistance and borrow large sums. etc. he needs at least a 20% extra buffer and cannot rely on precise calculation down to the dime.