How much I think I need to be financially free

Today I have some free time to write while waiting for my hubby to cook lunch, so am taking a rest and also doing some reading of other financial blog.

I was reading Seedly blog when I came across an article detailing how much savings one should have to retire at age 62.

For me, I plan to retire at age 40.

To derive how much I need a month, assuming I have retired and have no income, I first write down all my current fixed expenses:

1)  Insurance 450
2) Transport costs 80
3) Utilities 100
4) Telco and cable 150
5) Food 900
6 ) Groceries 300
7) Car loan 550
8) Costs associated for owning a car (petrol, car park, insurance, road tax) 380
9) Shopping 100
10) Exercise membership 170
Monthly total 3,180 = 3200 (rounded up)

Next, with this above list, I will go through each item and ask, how would this cost look like if I have retired.

1)  Insurance 450
2) Transport costs 80
3) Utilities 100
4) Telco and cable 150
5) Food 900  600 (I will be eating less expensive food at home)
6 ) Groceries 300
7) Car loan 550 Switch to taxi 320 (assume I go out every sat and sun and take taxi to and fro)
8) Costs associated for owning a car (petrol, car park, insurance, road tax) 380
9) Shopping 100
10) Exercise membership 170 (I would be exercising at home/park nearby)
Monthly total 3,180 = 3200 (rounded up) 2,020 = 2,000 (rounded up)

With this above, I know that I would need minimally 2k a month to retire. This means I need 24k passive income a year.

working backwards once again, I use 24k/rate of return from my investment (around 4%) = 600k.

This means I would need to invest 600k with a return of 4% every year, to support my retirement.

Currently I have already invested 250k. I would need 350k more.

With my goal of retiring by 40, this means I would need to invest 120k a year. Seriously, this is next to impossible because my annual income is less than 100k. So knowing that I can't do this, i need to be realistic about retiring at age 40.

Let say I delay my plan by 5 years, each year I would need to invest 44k (this is comparably more achievable). I then subtract another year

Retire at age 45 - invest 44k a year for 8 years
Retire at age 44 - invest 50k a year for 7 years
Retire at age 43 - invest 58k a year for 6 years
Retire at age 42 - invest 70k a year for 5 years
Retire at age 41 - invest 87k a year for 4 years

As I look at the figure above, I think i could aim to achieve retiring at 44. This is not something I could do easily, but it will be a challenge that I set for myself for the next 7 years counting from this year.