(edit: I posted coincidently with ziggy29, to this is not a response directly to the above post)
I wish I had it that simple. I'm retired, DW decided to keep working for a few years just because she wanted to. We have one kid in his last year in college, and one that is just applying to colleges. We're paying for that, so we have high expenses for another 4 to 5 years. I'm 54, DW is 49. my SS kicks in in 8 to 16 years, DW's SS in 13 to 21 years. Both with substantial amounts. DW has a significant pension that will start in 6 to 16 years. I just refinanced the mortgage at 4.375% fixed for 30 years, so that expense will disappear late in retirement. I would expect at least one of us would live that long. We'll have to find individual health insurance (if possible) when DW finally retires in maybe a year or two. Then we will transition into Medicare in 11 and 16 years respectively. After all that stabilizes, I think I'll be at something like 2% withdrawal rate, and nearly all will be coming from a Roth account, which also makes a big difference (hopefully still by then).
FIRECalc can still be run with most of that included, and I use my own program to calculate everything with taxes included, but my SWR varies nearly year-to-year. No way I can just get one number out of that. Wish I could! taking out a 4% each year would be much easier.