I'm not certain what you mean by common sense and basic math. I was SO happy to learn about the basics (here on the forum) of the 4% rule based on Bengen's et. Al's w*rk. I found that over 20 years ago and FIRECalc is what I used to decide I was FI back then.
I've always credited myself with "common sense" but I found the decision of whether I was FI or not was fairly complicated. I saw estimates of what one could safely withdraw as high as 8% or more for 30 years. (The financial "porn" of that era - ca 2002 suggested such withdrawals were possible.)
I had a direct report in my w*rk group who retired several years before I did because a local "guy" (broker) said he could give the guy monthly checks (essentially 8%) because he (the broker) would write covered calls on stuff in the portfolio. A few years later, my w*rk mate was repositioning cars to make enough money to pay his bills.
So, I credit this forum with most of my financial acumen. FIRECalc specifically is a great tool IMHO. YMMV
I did the spreadsheet thing for quite some time. After a while I stopped. Before stopping I did a parallel run for two years or so.
My basic focus is on the 'the gap'. The gap between what I receive each month/each year from various guaranteed sources and the our estimated cash flow in retirement. The cash flow is simple...a tape from our current account plus any upcoming changes to expenses/capital items. I do not care about expense categories....it does not change the bottom line cash flow number for me.
The next was investment income in the last year vs. our cash flow requirements for the following year.
I do a cash requirement calculation every November/December at the same time that I do a pro forma tax return for each of us. I calculate what we need and what we need for tax installments during the year and plan our equity withdrawals accordingly.
I compare investment income to estimated cash requirement (based on current and estimated future), adjusted for a conservative E&O percentage to cover off inflation, estimated tax rate, etc.
Those were finger in the wind educated estimates that took 15-30 minutes.
After that it was down to ensuring that the equity investment accounts are increasing at reasonable ROR, net of inflation, net of cash withdrawals.
As a result, I spend much more of my time on investment strategy and tax strategy. The parallel numbers between elongated spreadsheet and these fairly quick, common sense calculations have been surprisingly similar.