Well, I can't speak for most people, but one thing I've been doing is using this modeling as feedback into my budget.
1. Track my current expenses (I use https://lunchmoney.app/, but a spreadsheet works fine). Looks like I'm spending $X/month
2. Build a reasonably complete model of my financial life assuming $X expenses per month (I use ProjectionLab)
3. Do I run out of money before the end of my life? If yes, look at my expense breakdown and look for areas to reduce it (fewer ubers, eat out less, cut back on diamond-encrusted tiaras, etc). My new budget is $Y/mo. Goto 2.
4. Do I have a ton of money left at the end of my life? If yes, I can increase my budget a bit for things I like (more ubers, eat out more, more diamond-encrusted tiaras, etc).
I can revisit this over time as my income changes, or my retirement account value changes based on real-world market performance, or I get a windfall, or I get married, etc etc.
1. Track my current expenses (I use https://lunchmoney.app/, but a spreadsheet works fine). Looks like I'm spending $X/month
2. Build a reasonably complete model of my financial life assuming $X expenses per month (I use ProjectionLab)
3. Do I run out of money before the end of my life? If yes, look at my expense breakdown and look for areas to reduce it (fewer ubers, eat out less, cut back on diamond-encrusted tiaras, etc). My new budget is $Y/mo. Goto 2.
4. Do I have a ton of money left at the end of my life? If yes, I can increase my budget a bit for things I like (more ubers, eat out more, more diamond-encrusted tiaras, etc).
I can revisit this over time as my income changes, or my retirement account value changes based on real-world market performance, or I get a windfall, or I get married, etc etc.