From my point of view Withdrawal rate is pretty simple, "The percentage of your nest egg that you spend this year." However, I guess now there needs to be a qualifier,
" Nest egg balance at the beginning of the year.
I don't know where Withdrawal rate originated. My first notice of it is in regard to the Trinity study. And even with that it does start to get fuzzy.
Let's start with the $1M nest egg and the sustainable 4% withdrawal rate of $40,000 withdrawal the First year. Now it gets fuzzy if you stick with the Trinity study guidelines. The second year you add an inflation raise, say 2%, so you withdraw $40,800. But at this point it does not matter what the balance of your nest egg is, your withdrawal is calculated on the balance of the nest egg the first year you retired, plus an inflation raise for each year. Your nest egg may have increased or decreased, your withdrawal amount is still based on your first year plus inflation.
The fuzzy part, now if calculate your actual WD each year based on your actual nest egg it can be above or below the original 4% guide.
I'm not saying this method is ideal, I'm just saying the Trinity said, under most stock market scenarios, your nest egg will last 30 years if you follow that method.