FIRECalc does have a built-in percent of portfolio withdrawal option, including a setting for a minimum percentage of last year's withdrawal (such as for the 95% rule).
Set the withdrawal amount for the first year as normal. This sets the percentage amount that will be taken each year. Then click on the Spending Models tab and select Percentage of Remaining Portfolio. Within this selection you can also select a percentage of last year's withdrawal as a floor.
The results page gives the lowest portfolio amount throughout retirement:
"Here is how your portfolio would have fared in each of the 113 cycles. The lowest and highest portfolio balance throughout your retirement was $471,381 to $2,098,686, with an average of $1,118,338. (Note: values are in terms of the dollars as of the beginning of the retirement period for each cycle.) "
More relevant to the OP, the Investigate tab has a box you can check to generate an Excel spreadsheet for any one scenario starting on 1960 or after.
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Display the results of the retirement plan
The success rate of your portfolio and withdrawal plans, and optionally provide data and formulas in a spreadsheet format, using as the starting retirement year in the spreadsheet showing a full retirement cycle (must be on or after the first year data were available, and early enough to show a full cycle). Note: Spreadsheets are not formatted; they are for verifying numbers and calculations only. For 30 year terms only."
A lot of work, but you can get an exact number for your portfolio minimum during retirement for any scenario starting in 1960 or beyond.
Note that there is not really a "safe withdrawal rate" when using a fixed portfolio percentage withdrawal. You will never run out of money. Your key concerns are the smallest withdrawal made during retirement and the size of the portfolio, which are of course directly related. If the lowest portfolio value times your withdrawal percentage gives you a withdrawal that will meet or exceed your non-discretionary expenses you are in the "safe" zone. Of course many here would add a safety margin to that.