The way that I think of it is buckets. You never have a 40+-year period where your income and expenses stay the same, so it's better to break your life into chunks that reflect the different incomes/expenses over each time period, figure out how much you need for each chunk, and then add that together to figure out how much $$ you need at your retirement age to cover all of those buckets.
Example: say you want to retire at 55, will pay student loans until 60, and then plan to claim SS at 65. So start with the 55-60 bucket: say you need $5K/mo to cover your costs. Multiply by 5 years, that means that by the time you turn 55, you need to have $300K invested to cover that period. That sounds like a lot -- but remember you're not 55 yet. Maybe you're 45, which means your money still has time to grow. So maybe you need $150K now to get you to $300K at 55.
Next bucket is 60-65: at this point, your student loans are gone, so now your expenses decrease. So maybe now you need only $3500 to cover your expenses. That's $42K/yr x 5 years = $210K to cover that five years. But now that's even further in the future -- so maybe you only need to have $75K invested now to have $210K in 15 years.
Then at 65 you claim SS, so now you have another source of income. Your expenses are still $42K/yr, but now SS will cover say $22K of that. That means that now your investments need to generate only $20K/yr. So maybe you assume you will live until you are 90 -- so you need $20K/yr x 25 years, or $500K. But again, you don't need that now -- that's 20 years in the future. So maybe you only need $125K now to get you to that $500K by the time you are 65.
So if you add all those buckets together -- $150K + $75K + $125K -- that means you need about $400K invested now to cover your changing expenses and income from your targeted retirement date until you hit 90.
Obviously this is a hugely simple illustration that doesn't account for inflation, taxes, growth of your portfolio within each specified period (i.e., you don't actually need $500K the day you turn 65, because your investments will continue to grow over the next 25 years). But that is the concept, and it can give you a ballpark figure of how much you need to have invested to make it through. Note also that the way I did the math is helpful to tell you how close you are now, but you can also do the math based on the $$ you need to have invested on the day that you FIRE, because that will give you a better target for your savings while you're working to get there. And you don't need to be a math whiz -- I just use basic calculators on the web (e.g., google "how much do I need to have saved to have $500K in 15 years?" and follow the links until I find one that gives me the inputs I want).
