Percentage of ripe levain used in building a levain for a formula

When I made Dan DiMuzio's Baguettes with Liquid Levain, I was puzzled by his formula for building the liquid levain.
100 g bread flour 100 g water 66 g ripe levain
This formulation yields 200 g ripe levain plus an additional 66 g of levain leftover to build the ripe levain for the next batch.
What puzzled me was why Dan chose 66 g so I asked what was so magical about that number. Dan's reply was most interesting so I asked him if he would mind making the response on a post that we all could read.

1:3 ratio; I understand. How do you decide what that ratio ought to be? Some of your formulas are 1:4 others are 1:3. I really want to understand the basis for making such a decision. It can't be based on the possible sluggishness of a starter; I always assume I am working with a healthy ripe starter.