Adjusting as you go. You need 2 million in today’s dollars to fun 80k in today’s dollars, you save based on today’s dollars and hopefully adjust every year to keep up with inflation and changes.
You would hope people arent just planning once (or not at all) and just spitballing some random number.
Op shouldn’t set a goal for 1 or 2 million and then get there in 20 years in 2040 dollars and realize he has half as much as he needs. Gotta track your spending and update your numbers at least once a year and make sure you are on track.