Have made plenty of mistakes, but if there were to be one suggestion that I would make to avoid a big mistake, it would be to do Detailed Planning... and not stop at a calculator to determine if a plan would work.
Of course, using Firecalc or any other measure of future dollars is important, since they include variables that would be almost impossible to figure from scratch.
That said, output is only as good as input, and garbage in = garbage out, is the key.
Short cutting actual plans by using expense numbers @ 70% of ending salary or some other general formula is a bit of Russion roulette, IMHO.
For starters... because we're all in different circumstances here, real numbers are important. If your worth includes a $1Million House, and a $150K Sailboat and you're used to international travel, then your priorities are different from one who is using Social Security as a major and necessary supplement to retirement spending.
I would iterate my original plan, which was the creation of many spread sheets/calculations... with a projected net worth final amount for each year of retirement. For practical purposes, did not include inflation, but DID calculate a nominal investment return rate. In my case, 24 years ago, 2%. In our case, we were at the lower end of the net worth scale, so things like housing value, Social Security, and some degree of detailed cost of living expenses were important. That was the basic plan.
Next was a hypothetical year by year calculation... including:
Cost of healthcare before medicare... then after.
Selling original home, the buying "down". for 5 years.
Plan for major purchases... Cars, Travel to look for southern home.
Later years, buying "final home"
Real estimates for house taxes, fees, furniture.
Budgets for food, eating out, sin expenses etc.
Entertainment, travel, .. higher in early years.
Long term care insurance
... and some substantial planning for those "occasional" expenses, like major repairs, high dental bills, help for family.
Large green spreadsheets... dozens... planning out a hypothetical life for 30+ years... Not really a plan, but a "What IF" outline. Something we never really used.... but a guide that prepared us for the long haul. We changed, dozens and dozens of times, but in every case, it was something that we knew about, and knew how to adjust when necessary. From the very beginning, we slept well and never looked back in worry (except one time when we thought we might be on the hook for a few hundred thousand in medical expenses... that didn't happen).
The Firecalc numbers work out for me, but the detailed planning is what we used to make it work.
