They take the 85% of 100k because it's assumed when you retire, you won't be saving any of your income, just drawing down.

This calculator is often uses to demonstrate different savings rates : https://networthify.com/calculator/earlyretirement

The reason why saving more quite rapidly lowers your retirement age is because it's a twofold saving. Firstly you are saving more money which is good, and secondly you are learning to live on less.

To take a super simple example. Let's say you can live on 50k a year. And you can get 3.5% on term deposit rates (You can in NZ). Then you should need 50,000 * (100/3.5) = 1.4 million approx to retire and be able to earn 50k a year off interest alone.