Start with the gap between the spending you expect and the regular income you have already estimated. That gap is what your savings would have to cover.
An example with numbers
If you spend $1,800 a month and expect $1,300 of monthly income, $500 is missing. With $90,000, a fixed $500 withdrawal would last 180 months, or 15 years, with no returns, inflation or taxes.
Try it with your numbers
Repeat the calculation with a $650 gap. Then note which assumptions you would need to project spending and income in money of comparable purchasing power.
This calculator is on its way. In the meantime, you can do the calculation on paper or in a spreadsheet.
Example figures for learning purposes. They are not a recommendation and do not replace professional advice.