As I think about variations on the 4% rule the more I come back to a different way to think about readiness for retirement.
Start with the expected number of years to live in retirement.
Multiply that by current expenses/spending.
The result is how much you need in retirement funds (across all sources to include pension, SS and investment, etc.). Reality may be more or less, depending on inflation, investment returns, and actual spending. All of these are unknowns, but we can use the same simple arithmetic (expenses x years) to evaluate the situation each year.
This approach works both before retirement, and in the middle or end of a long retirement where the 4% doesn't really apply.
I like your thinking. This is a great back of the envelope math.