When thinking about years of work until financial independence, its worth understanding the impact that savings rate has - the fraction of post-tax income remaining to invest in growth assets after expenses. If you're not familiar with savings rate, here's a simple calculator to help build intuition. The assumptions of asset returns & portfolio required to sustain retirement are a bit optimistic, but it's directional…
One thing I have wondered about these calculations: is one's annual income supposed to be inclusive of all remunerations? You know, things like employer matches in retirement accounts, health insurance contributions, etc.?
seems completely reasonable to model this as income.
> health insurance contributions
If your employer provides a health insurance benefit that would otherwise cost you $X / yr, you could model that as +$X / yr income and +$X / yr expenses.
Simple FIRE calculators like that Networthify one assume that your expenses in retirement are the same as when you are working, but that's probably not true for many people, for numerous reasons: health insurance once you're not longer working, paying off a mortgage, children launching into the workforce, increasing health expenses as you age, lifestyle changes when you're not working, potentially moving to a different area with a different cost of living.
But as a 0-th order approximation, assuming expenses in retirement are the same as when you are working is a place to start when assembling a rough plan.