A withdrawal rate is an assumption for exploring scenarios. Small changes in annual spending can change a lot the starting capital that assumption would require.
An example with numbers
To withdraw $18,000 in the first year with an assumption of 3% of the starting capital, the arithmetic target is $600,000. With $21,000 of spending it would be $700,000. The division alone does not guarantee the money lasts through retirement.
Try it with your numbers
Repeat with two spending levels and two hypothetical rates. Keep the calculated starting capital separate from a simulation of withdrawals, inflation and variable returns.
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.