How do you calculate how much you should invest/save for retirement?

Working backwards will give you an easier answer. For example, I wanna retire by 40 years old with $1m assets generating 6% dividend per year. that will give me $60k per annum and $5k per month.

if i have $300k now, I'll need to save $70k a year for 10 years given the fact that I'm 30 this year. Something alone that line. :)

(1) Get a sense of how much you will need monthly when you retire (in today's dollars). Include the cost of insurance at your (advanced) age, a buffer/rainy day fund, as well as other oft-neglected parts (the occasional phone purchase, snacks etc). Let's call this $X / month

(2) If you've read the FIRE movement, you will notice that they advocate the 4% rule - basically they believe that over the long term, your investments should be able to give you 4% returns per annum without you touching the principal amount (aka, 4% in perpetuity). This means you need 25X of 12 months of your $X. For example, if X=$3,000/month, you need 25 X 12 x 3,000 = 900,000 (in today's dollars)

(3) Work on a plan to accummulate this amount, and remember to take into account inflation (900,000 today is worth more than 900,000 10 years from now)