I don’t think there is a single perfect metric for variable withdrawals — that’s probably part of why people like ERN dislike them because they are harder to think and reason about. Much easier to just say “withdraw 3.25% forever”.
Instead there’s a grab bag of numbers that all tell you something slightly different about variable withdrawals. I think of it like looking at a sculpture: to really see the sculpture you can’t just stand in one place, you need to move around.
The “big three” that I look at Certainty Equivalent Withdrawals (CEW), Withdrawal Efficiency Rate (WER), and Harvesting-Rate Efficiency Metric (HREFF). Another one I’ve started looking at what I think of as “the Ulcer Index for Withdrawals”.
Maybe I’ll write a post on them, since this comment is already getting long :)