> How much income will you need...?
I tried to get my own answer for this question in a couple of ways (retired 5 years ago, age 62 so not early).
First, I tracked my expenses, but didn't have a planned budget. I wanted to know how much I spent and where it went. I kept monthly and yearly summaries on a spreadsheet and made charts to better visualize trends and variability. I think just tracking expenses was encouragement to live modestly and contribute regularly to savings: information not coercion.
Second, I tracked potential future income/benefits. I have an employer retirement plan and an IRA plan. Before retirement, I used the retirement plan's calculators to estimate that benefit. For my IRA I used a 4% withdrawal estimate.
Third, I assumed my expense totals would be the same in retirement. I might spend less on commuting, work clothes, etc., but more on medical care, vacation travel, and so on. I estimated housing costs for either renting or having a mortgage. (Ended up selling the house when I retired, moving and renting for 3 years, then buying a new place and getting a small mortgage for that.)
Over the short term, assumptions of continuity are reasonable. As you point out, the longer term is much more of a question mark. So, fourth, I tried to make allowances for uncertainties and contingencies. How much padding or margin did I want to be comfortable? Perhaps 10% to 20% more income than one's average expenses would feel comfortable? Perhaps a contingency plan for part-time work?
All these numbers will be in current dollars, so they will be comparable. What I discovered when running my numbers (after learning about an opportunity to buy some retirement plan benefits with money I had saved in a 401-K) was that I could then afford to retire. I can't predict the future either, but felt comfortable with the uncertainties I could see.
Following one's own situation roughly once a year, one can see how different conditions and assumptions give different results. Generally, the less one spends and the more savings one has, the more flexible one can be in choosing jobs, places to live, lifestyle, and so on.