See what I wrote about depleting the principal. $3m would plan for leaving that entire sum available “after-120”. $2m would plan for leaving $0 after 16/17 years.
The math for such a strategy is simply dependent on both the earnings rate on the money and the draw down rate.
The $2m I quoted was for a male age 65 receive $10k per month in perpetuity. No running out after x years. That's the contract. My point was that if someone plans to leave the principal alone and live on the earnings, they'd do better to buy a lifetime annuity. Obviously it's not apples to apples for various reasons, but the point stands.