How much do you need to accumulate to retire at 65 years old and have $3,500 to spend a month for 20 years

Came across a most recent article dated 5 Dec 2014 on "Majority unprepared for retirement: Survey" which says that (my summary):

- THE majority of Singaporeans do not have a financial plan for retirement, are afraid of planning ahead and need more than they estimate to retire, a new survey has found.

- Many respondents hope to retire comfortably but only a minority are actually taking tangible steps to meet that goal.

- Singapore government estimates show that the number of residents aged 65 and above is expected to triple to 900,000 by 2030.

- Many Singaporeans do not even know where or how to start planning.

- 23% feel that they do not have enough funds to start investing.

- Workers and retirees agreed that Singaporeans do not know enough about building up a nest egg.

- The survey also found that people are underestimating the funds they need for old age.

- About 85 per cent of respondents said they hoped to have about $3,500 to spend a month for 15 to 20 years after retiring at the age of 65. They believe that it would be sufficient to set aside about $480,000 to $700,000.

- calculations by DBS financial experts showed that for monthly payouts of $3,500, Singaporeans will need a $900,000 fund.

You can read the original article at this URL...

   
So, how much really should you need to accumulate at 65 years old to retire with $3,500 to spend a month for 20 years?  Obviously, the actual figure will depend on the assumptions that the estimator made, such as:
1) living cost inflation in next 20 years
2) The return from the capital you have at age 65 for the next 20 years. 

Without these 2 assumptions being clearly made known, people are just groping the elephant while being blind-folded! 
So what are reasonable assumptions for living cost inflation and return on capital? - These have a lot to do with government policies, and Singapore and the world's economic situations, and also cost of oil and Singapore's currency exchange rate (strong S$ or weak S$). 

In general:
1) A stronger S$ will usually mean lower living cost inflation (since most of the items needed are imported). 
2) Lower oil price will usually mean lower living cost inflation (since oil is used to produce electricity and run almost all transport and used by almost all businesses). 
2) A stronger and appreciating property price will mean more comfortable retirement for most Singaporeans (since >90% of Singaporeans own their homes and they can downgrade to extract cash value out of their homes for more comfortable retirement). 

Let's just assume that living cost inflation is 3% and return from capital is 4% (CPF retirement fund payout rate), then in order to have enough to spend $3,500 per month for the next 20 years, a person will need to accumulate:
$925,000
based on my calculation. 

However, really, do you really need $3,500 per month to live in Singapore? 
I don't think so!
Actually, I believe $2000 per month is sufficient to live in Singapore, assuming that you have your house already fully paid up and the full sum of $2000 is just for month expenses! 

Ok, let do a scenario study assuming 3% inflation and 4% return, how much do a person need to accumulate to retire for 20 years for these estimated living expenses per month? 

Living expenses per month        Amount needed at 65 years old
$1000                            $265,000
$2000                            $530,000
$3000                            $795,000
$4000                            $1,160,000           

Note: My above calculation assumes that the expenses drawn down increases with inflation and not a flat figure for next 20 years since I assume that expenses will increase as inflation increases.